With considerable sorrow we can add Rodney Dangerfield to the roster of those who have Just About Had It. In the opening monologue of his ABC comedy special "Rodney Dangerfield Exposed," at 8 Sunday night on Channel 7, there is no new material whatsoever, and the slender sketches that follow are painfully unfunny.

Rodney needs emergency rejuvenation. The sketches include one in which he clears out a party by offending all the guests. It would be funnier if Rodney weren't beginning to seem the performing equivalent of the seedy boor he plays. Among those dragged along for the crawl on this outing are Harvey Korman (a great natural resource repeatedly wasted), Morgan Fairchild (who seems to have a knack for sly comedy that can be glimpsed beneath the bad material) and that tiresome lunk-jock act of Bubba Smith and Dick Butkus.

One sketch, a spoof of "Splash," depends on the audience's being amused when people throw a baby out a ship's porthole. Later, Rodney plays a man who falls in love with an inflatable woman, a comic idea previously executed with much more hilarity by Benny Hill. Finally, the show evidences a trace of life when Rodney joins a chorus line of dress-alikes in red tie and drab black suit, but by that time Rodney has become decisively over-"Exposed."