In so many ways, it all seemed quite ludicrous. Only a second-half snooze by the Washington Redskins allowed their blowout to blow back in.

The Redskins defeated the Minnesota Vikings tonight, 31-17, in a game that really was so one-sided that offensive tackle Joe Jacoby and defensive tackle Darryl Grant each scored second-quarter touchdowns, both on fumble recoveries.

Perhaps the most concerning matter of all for the Redskins was raised after the game. That's when running back John Riggins, who did not play because of continued pains in his lower back, told a team official that he would enter Sibley Hospital in Washington Friday for testing and perhaps traction.

Riggins said, "The most discouraging thing is that I've been resting and still there's no improvement. I wouldn't go through another year like this, but that doesn't mean I'm announcing my retirement."

How one-sided was tonight's game? It was 31-0 by halftime. On the first play from scrimmage, Joe Theismann threw a 68-yard touchdown pass to Calvin Muhammad with nary a Viking in sight.

Despite a 17-point second-half rally by the Vikings, pity the 54,017 Minnesotans who saw their Purple People eaten once again inside the Metrodome. Never before in this 1984 season has a team made Buffalo seem so good.

In fact, Theismann finished by completing 19 of 24 passes for 223 yards and two touchdowns. And Muhammad caught five passes for a career-high 115 yards. Further arithmetic shows that, over the past two games, Theismann has completed 45 of 57 passes for 534 yards.

So the Redskins are 9-5, moving three days ahead of Dallas and the New York Giants (both 8-5) and into sole possession of first place in the NFC Eastern Division. To keep pace with the Redskins, Dallas must beat Philadelphia and the Giants must beat the New York Jets on Sunday.

The Redskins next will play Dec. 9, at Dallas. Riggins was sure to note, "I'll try to play against Dallas. It will have to be something real major to stay out of that."

"The 'Skins are happy," Coach Joe Gibbs said. "It's good to be 9-5. I think this came at an important time for us."

And what about the second-half doze? Said Gibbs: "I don't think we were flat. It is just that Minnesota made some plays on us and I think they did a good job."

There seemed no way that the Vikings (3-11) would keep pace with the Redskins tonight. These are the same Vikings who had given up an average of 40 points and 450 total yards over their last three losses to Green Bay, Denver and Chicago.

But the Vikings sent shivers up Redskins spines by rising from their pathetic state in the second half behind quarterback Archie Manning, the 14-year veteran.

Manning replaced ineffective Wade Wilson in the second half and threw two scoring passes to Leo Lewis. And 42-year-old Jan Stenerud kicked a 31-yard field goal.

Meanwhile, the Redskins' offense began to struggle. The pass protection seemed to vanish for Theismann, who did manage 13 consecutive completions in the second half.

Minnesota wasn't finished for good until 2:30 remained. That's when the Vikings, trailing by 31-17, had driven to the Washington three and had the Metrodome a'roarin'. Allen Rice ran right, was hit by defensive end Dexter Manley and fumbled at the one. The ball bounced into the end zone and out of bounds for a loss of possession and, it turns out, a loss of game.

"When we were down, we needed to get motivated," said Lewis, the receiver who caught seven passes for 130 yards. "We needed to put points on the board to show the world that we can be competitive. When you're down, 31-0, you let it all hang out."

Manning, who completed 11 of 22 passes for 164 yards in the second half, added, "We went out and just let it rip."

Vikings nose tackle Charlie Johnson said, "A few more minutes and we could have made something happen on national TV."

Although a 32-point Vikings comeback victory seems improbable even in retrospect, Minnesota's comeback followed the pattern of the Redskins' recent victories over Detroit (28-14) and Buffalo (41-14). At a late juncture in those games, too, a Redskins rout came back to haunt Washington.

The Vikings outgained the Redskins, 370 yards to 346, which is quite a turnaround when you consider that the Redskins outgained the Vikings, 201 yards to 73, in the first half.

Furthermore, the Vikings gained 170 yards rushing, 133 of them in the second half. This included a 39-yard run by Darrin Nelson and a 36-yard run by flanker Mike Jones.

The first-half trend was set 18 seconds into the game when Theismann passed to Muhammad deep downfield, on the left side. Muhammad outran strong safety Tommy Hannon for the touchdown. Muhammad also ran past a banner along the way that read, "It's About Time to Give Bud a Call."

Times have been tough since Les Steckel replaced Bud Grant as coach this season. Bumper stickers around the Twin Cities read "Less Steckel."

By the end of the half, this was what the Redskins had done:

*The defense had limited the Vikings to 73 total yards. Remember, though, the Vikings played without three injured stars: quarterback Tommy Kramer (shoulder), running back Ted Brown (shoulder) and wide receiver Sammy White (lacerated tongue). Brother, were they missed.

In the first half, Redskins defensive tackle Grant scooped up a fumble by Wilson and returned it 22 yards for a touchdown and the 31-0 lead with 7:44 left in the half. Blitzing Neal Olkewicz had sacked Wilson, causing the fumble.

Prior to this, linebacker Rich Milot and strong safety Ken Coffey each intercepted Wilson. Milot set up a touchdown and Coffey ended the Vikings' only real drive in the first half, at the Washington three.

*The Redskins seemed to move with ease in the first half. Soon after Milot returned his interception 27 yards to the Vikings' 12, Theismann threw a four-yard scoring pass to tight end Clint Didier (17-0, with eight seconds left in the first quarter).

Then came Jacoby's big moment. Rookie rookie running back Keith Griffin ran 13 yards, then fumbled at the Minnesota seven.

The ball rolled into the end zone, where Jacoby recovered it in a pileup. He celebrated the first touchdown of his career with a spike. Why not? The Redskins led, 24-0, and only 19 minutes had been played.

The game had become so one-sided that someone in the press box wondered aloud if the television monitors in the press box might be switched to "Hill Street Blues."

The Vikings' offensive and defensive lines have been a cause of much concern this season. Injuries have caused inefficiency in both units. The offensive line had yielded 51 sacks and the defense had only 20 sacks before tonight.

Poor Wilson. He completed only eight of 17 passes for 63 yards in the first half and was sacked three times. When Wilson wasn't being sacked, he was being pressured.

This Redskins' victory, boosted by the return of running back Joe Washington (a team-high 56 yards on 16 carries and one lost fumble), did have a record book relevance.

With his 11th completion tonight, Theismann claimed his third club record in three weeks from Sonny Jurgensen. Theismann passed Jurgensen's mark of 1,831 completions in the third quarter; he now has 1,839.

And kick returner Mike Nelms set his eighth club record, slipping past Dickie James with 3,959 yards in kickoff returns.

By scoring two touchdowns on fumble recoveries in the same game, the Redskins tied a mark held by many teams.

"This gives us a chance to take a blow," Gibbs said, "and regroup for the rest of the season."