Brace yourselves, "Twin Peaks" fans. (Both of you still out there).

The show is as dead as Laura Palmer. And Maddie Ferguson, for that matter.No, there hasn't been any official cancellation notice from ABC. And, yes, as far as we know "Peaks" will continue to air on Thursdays at 8 p.m. for the next five weeks.

But after that, the show will disappear forever.

And it wasn't BOB who killed "Twin Peaks" - it was lousy ratings. The return to Thursdays was a last-ditch effort to revive the show's anemic numbers - and that effort failed miserably.

Last week, "Peaks" didn't just finish behind top-rated "Cheers." And it didn't just finish third behind "Cheers" and CBS' "The Antagonists."

"Peaks" finished fourth, behind those two shows and Fox's "Beverly Hills, 90210. (See if you can explain that, Agent Cooper.)

Before the show got pulled off Saturday nights, it was averaging a 7.5 rating and a 14 share. (A rating point equals 931,000 homes and a share is the percentage of homes actually viewing television that are tuned in.)

Well, this past Thursday "Peaks" dropped off to a 6.2 rating and a 10 share. Translated, this was almost as bad as Leo Johnson's attitude.

So don't bother to write any letters to ABC. It won't help. Just get ready to say good-bye.NBC RENEWALS: The news is considerably better for a couple of excellent - but not highly rated - series on NBC. The Peacock has picked up both "Quantum Leap" and "Law & Order" for the 1991-92 season.

The order for "Law & Order" isn't exactly a surprise, but was by no means a certainty. The well-made drama featuring both police detectives and district attorneys working on the same cases ranks 46th out of 129 shows so far this season.

("Law & Order" is, however, going on hiatus after tonight's episode.)

But "Quantum Leap's" future was doubtful at best. Ranked just 83rd so far this season, the show was dying on Friday nights, then spent several weeks in limbo before resurfacing in its old time slot, Wednesdays at 9 p.m.

So let's hear a bit of a cheer for NBC. (Although a cynic might point out that the network has had such a rough season it can't afford to cancel any marginal shows.)ABC PROGRAM CHANGE: As of April 17, "Gabriel's Fire" will replace "Equal Justice" on Wednesdays at 9 p.m. on ABC.

That's good news for the James Earl Jones vehicle and bad news for the drama about a group of district attorneys.

For "Fire," which stars Jones as an ex-con now working as a private investigator, it's a vote of confidence from the network. For "Justice," it's a bad sign - there will still be several unaired episodes remaining when the series heads off into the limbo of hiatus.

Neither show has had much success in the ratings this season, and neither is a terribly strong candidate for renewal.

Indications now are that "Gabriel's Fire" is a considerably stronger candidate than "Equal Justice."BYE, BYE BONET: There's even more good news out of NBC lately - that dreadful Lisa Bonet won't be returning to "The Cosby Show" next season.

You'll remember Bonet. She plays daughter Denise, the strangely attired and coiffed one who has done little but whine during her time on both "Cosby" and the first season of "A Different World."

Bill Cosby blames himself for Bonet's departure. He maintains she's being dumped because her character has never been allowed to grow up.

(Of course, he could also take the blame for casting her in the first place.)

Strangely enough, Denise's stepdaughter, Olivia (Raven-Symone) will return for the show's eight (and apparently final) season. No word on the fate of Denise's husband (Joseph Phillips).SADDLE 'EM UP - AGAIN: "Gunsmoke," which was the longest-running prime time series in network television history (1955-1975), hasn't reached the end of the trail yet.

CBS has announced that James Arness will return as Matt Dillon in "Gunsmoke III: To the Last Man." (There have been two other "Gunsmoke" movies - "Return to Dodge" in 1987 and "The Last Apache" last year.)

This time around, Matt and his daughter (Beth Stock-Poynton) get caught in the middle of a bloody range war. There's no airdate yet.

Gee, you don't think the popularity of "Dances With Wolves" has convinced CBS there's a market for Westerns, do you?SULLIVAN SURFACES: Kathleen Sullivan, the one-time KTVX anchor who's been looking for work ever since she was unceremoniously dumped by "CBS This Morning" just over a year ago, has finally found it.

She'll join Don Criqui and Ahmad Rashad as a studio host for NBC's cable pay-per-view coverage of the 1992 Summer Oympics.

Oh, and she did have one job in the last year. She had a cameo as Frank Fontana's date in an episode of "Murphy Brown."