The Redskins mailed a "Dear John" letter today. By trading for Wilbur Jackson, they have told John Riggins they can get along without him. If the fugitive fullback wants to keep plowing his farm in Kansas, that's fine here. This Dear John letter told Riggins to stay on his John Deere.

But without Riggins, as today's trade makes palpably clear, these Redskins are desperate. They won't renogotiate Riggins' contract as a matter of principle, saying he signed a fair contract and they expect him to live up to it without the pot being sweetened. Principle can be expensive, however, as we learned again today when the San Francisco 49ers stuck it to the Redskins.

To get Wilbur Jackson, the Redskins are said to have given up two No. 2 draft chioces, one next year and another in 1982. So they have given away a nice chunk of the future to pick up an old running back who may have left his future on an operating room table two years ago.

As good as Jackson once was, and as bravely as his coach talked of a 1,000-yard season this year, the fullback will be 29 before Thanksgiving and he is coming off a 375-yard season. The year before, he didn't play at all because of knee surgery.

Still, the deal for Jackson had to be made whatever the cost to the Redskins. If you're drowning and someone offers to sell you a lifejacket, you don't haggle long over the price. The cost of Jackson is a fine example of price-gouging, heretofore practiced best by the oil companies. And although bound by agreement to keep the sale price secret. 49er Coach Bill Walsh said the draft choices were "high, high."

And ho, ho," he might have added.

The Redskins have put the best face on the barter, saying Jackson proved last year he can last an entire season on the repaired knee while working behind an offensive line that couldn't knock down the Lennon sisters. Once when he was young, and even then on terrible 49er teams, Jackson was a 700-yard runner -- and the Redskins think their offensive line can restore Jackson to the frolics of boyhood.

"It was not unusual for Wilbur Jackson to make a 50-yard runs," said Fred O'Connor, a Redskin assistant coach who was the 49er head coach the year Jackson sat out. "And he had a couple of 90-yarders three years ago. I'm sure his speed is about the same -- he did 4.6s in the 40. He's an elusive type runner who is a good pass receiver, a solid blocker strong enough that you can't arm-tackle him."

At 6-foot-2 and 219 pounds, Jackson is the big back the Redskins are missing as long as Riggins drives his tractor. At 5-11 and 210, Clarence Harmon has been the biggest frontline back in the Redskins' preseason lineup. While Harmon has had productive seasons in his role as Riggins' backup -- a role that included important dramatic parts on nearly every third-down play -- he does not have the size and strength to carry his quickness a full season as the only fullback.

Besides, no coach wants to go into an NFL season with a backfield that contains no runner who gained more than 472 yards the year before, Without Riggins, that is the Redskins' position. Those 472 yards were Benny Malone's -- and Malone is likely to be cut from the team soon.

From San Francisco come reports that Jackson has been terrific in training camp. The 49ers' first draft choice was a fullback, Earl Cooper, and Jackson rose to the combat. He scored a touchdown in each preseason game (which gives him twice as many touchdowns as the Redskins).

Walsh, the 49er coach, said before camp opened that Jackson could be a 1,000-yard rusher this season. He hoped so, anyway, because San Francisco, or any other team for that matter, isn't going to be much without a horse running loose every Sunday. That's why Jack Pardee, the Redskin coach, says, "We hope Jackson can fill Riggins' role here."

An hour or so in close company with an NFL book of 1979 statistics reveals the importance of the 1,000-yard rusher.

Of the 10-playoff teams and Washington (the only 10-game winner not to make the playoffs), eight had someone over 1,000 yards.

Conversely, of the 10 NFL teams with losing records, only two had 1,000-yard rushers.

Of the 10 playoff teams, the three without 1,000-yard men all lost in the first round.

Anyone who is made dizzy by numbers is allowed to get off here, or, pretty please, skip ahead to the words-only sentences near the end. Here, then, are numbers that demonstrate how fragile the Redskins' success was last year and how the absence of John Riggins could bring the whole thing tumbling down.

Of NFL's 10 teams with a 10-6 record or better, the Redskins ranked eighth in total offense, ahead of Chicago and Houston.

Of those 10 teams, the Redskins ranked last in total defense.

The 1,000-yard rusher is more than just a good runner. He is a symbol of a team's offensive health, for without a balanced passing offense and without a stout offensive line, Bronko Nagurski couldn't gain 1,000 yards today. Riggins' 1,153 yards were the product of a wonderful athlete helped by a lot of friends.

Which brings us to so exotic a statistic that even the most obsessed filbert will faint away in admiration: Yardage Differential. This number proves that a team without a dominant runner is a team without much hope.

Of the National Conference's five playoff teams and Washington, all had dominant runners who each gained at least 490 yards more than the team's second-best man. The average yardage differential was 770 yards.

Conversely, the eight teams at .500 or below had only two runners clearly dominant (Ottis Anderson, William Andrews). Among the losing teams, the yardage differential average was 390.

Now it is clear why the Redskins are despearate. Already haning into the playoff race by the fingernails of a defense that ranked last among contenders and an offense better than only the Bears and Oilers -- already scratching and scuffling, they now have to do it without the 1,000-yard dominant runner who prefers to sit at home rather than play two more years for $630,000.

Or does he really want to pass up that money?

Larry Brown, as marvelous a runner as ever wore the Redskin livery, tried to hold up George Allen for more money in 1973. He stayed out of camp, pouting.

Allen did this in response: He signed up Duane Thomas, a fair runner himself.

Five days later, here came Larry Brown to training camp.

If the Redskins' deal for Jackson was expensive, it will be worth the price should Jackson turn into the 700-yard runner of his youth -- or if his presence convinces John Riggins that the show is about to go on without him.