By MICHAEL QUINN

It could be the weirdest tribute that Johnny Carson will receive in his farewell week. Daytime gab-queen JOAN RIVERS, on the outs with the Tonight show titan since her ill-fated 1986 effort to launch a competing late-night program, opens her Monday salute to Carson by weepily denouncing her exile from the guest list during Carson's closing days ("It's just like the Stalin purge!") -- and complaining, "They wouldn't let me show a clip. We asked over the years, and they keep saying, `We will sue you.'" Actually, control-freak Carson nixed similar requests from several shows -- but Rivers wasn't one of them. Her producers made no effort to obtain clips, knowing what Carson's response would be to that familiar inquiry,"Can we talk?"