"We Want the Islanders" reverberated across Madison Square Garden in the third period tonight as the suddenly revived New York Rangers consigned the second-place St. Louis Blues to the same scraphead on which they had previously dumped the fourth-place Los Angeles Kings.

The dream of rabid Ranger fans, a playoff duel with the Stanley Cup champion Islanders, became a reality after the Rangers ripped the Blues, 7-4, to wrap up that quarterfinal series in six games, and the Islanders accomplished the same in Edmonton, 5-2. The series begins Tuesday.

The Rangers, 13th during the regular season and 33 points behind the Blues, jumped to a 4-0 lead in the first 22 minutes tonight. Then St. Louis retaliated with three goals in 1 minute 50 seconds to get back in contention.

Rookie Lance Nethery produced the big goal with 2:57 left in the second period, rebounding his own shot that had struck a post to make it 5-3. A sudden turnabout in the third period wrapped it up, as Ranger goalie Steve Baker made an outstanding save to rob Bernie Federko of a hat trick and Peter Wallin scored at the other end on a shot that carmored off the skate of St. Louis defenseman Rick Lapointe, making it 6-3.

"After that fourth goal, mentally maybe we thought we had the game," said Barry Beck, who scored the fourth goal and was a mainstay of the Ranger defense. "We thought they'd roll over and die but they kept coming. If they'd got a couple of breaks when it was 4-3, we might be going back to St. Louis.

"That was a big goal by Lance. We were just hanging on at the end and i'm glad we'll have the weekend to rest. I know I'm tired and I think our other defensemen will tell you the same thing."

Steve Vickers put New York ahead after 3 1/2 minutes and then a fight between Ranger Ed Hospodar and the Blues' captain, Brian Sutter, shoved St. Louis farther behind the eight ball. Each was penalized 17 minutes, and since Sutter is a first-line 35-goal scorer and Hospodar a physical player of little talent, the result hurt St. Louis. Hospodar initiated the brawl during a line change, shoving his stick into the chest of Federko, who backed off, to be replaced as a combatant by Sutter.

Linesman John D'amico, trying to separate the two, was struck in the chest by a helmet and suffered bruised ribs. He was taken to Lenox Hill Hospital for X-rays that proved negative, and there was a 20-minute delay while sub Ron Fournier suited up.

New York scored twice more before the period ended. Anders Hedberg putting one through St. Louis goalie Mike Liut's pads and Ron Duguay scoring short-handed. Hedberg was off at the time for interference on one of the few penalties referee Wally Harris chose to call in a game filled with elbows, high stick and other assorted fouls. Harris took one of the few clean checks of the first period, when the Blues' Gerry Hart flattened him while pursuing a straight-line route to the puck carrier.

To add to the Blues' woes, an apparent goal by Federko was disallowed by Harris, who ruled Federko struck the puck with his stick above his shoulder.

Seventeen seconds after Beck made it 4-0, Federko put the Blues on the scoreboard. Lapointe and Federko scored two more before Baker was able to make a save in the second period. When Ranger Ron Greschner was banished for tripping 37 seconds after Federko made it 4-3, there was panic among the 17,384 fans. Baker, however, made two good saves to help kill the penalty and the Rangers regrouped.