Here's some definately not in the same t.v universe:
ALF has a dream sequence about Gilligan's Island years later with Bob Denver, Russel Johnson & Dawn Wells playing their own roles pay a visit.
Roseanne had a similar dream sequence in 1995 about Gilligan's Island with her as Ginger(!) & the rest of her family as the G.I cast ON THE ISLAND, then when closing credits are rolling it has the REAL Wells, Louise, Denver & Johnson sitting in the Roseanne living room set with roles changed to that of Roseanne cast. I guess it still counts even if it's a dream sequence!
more...
Andy visits Gomer on U.S.M.C
Gomer comes home & visits on Andy Griffith(or was it Mayberry R.F.D)
Seinfeld went on with Jay Leno
Kramer went on with Regis & Kathy Lee (with his coffee table book that folds out into a coffee table)
(not really another show) but years later Al Molinaro (Al) from Happy Days appears as his old character in Wheezer's "Buddy Holly" video. "Fellas, how was the fish?" "Not so good, Al" "Not so good Al...yep, yep, yep, yep..."
Don Ho had his own show in '76 & appeared on Sanford & Son's 2 part Hawaii episode.
"Hawaii Five-0 Theme" by the Ventures was also used but no Kam Fong as Chin Ho or Jack Lord either.
After "Daria" began she appeared back on Beavis & Butthead.
In 1996 Arnold & Mr. Drummond paid a visit (as their Different Strokes characters) to the "Fresh Prince of Bel Air" show.
"What chu talkin bout WILL!"Uncle Joe Carson played by Edgar Buchanan was the same character on Petticoat Junction, Green Acres AND a couple episodes of the Beverly Hillbillies (do I get bonus points for 3?)

Pat Boone was on Beverly Hillbillies as himself once.