Diagnosis Murder had one or possibly two.
Mannix appeared on a Diagnosis Murder episode well after his series was done.
Matlock also appeared on Diagnosis Murder but I’m not sure if it was during the Matlock run or after.
After the cancellation of The Golden Girls and its sequel, Golden Palace, Sophia Petrillo appeared on Empty Nest and Nurses.
Beat ya. 
And yes, Matlock appeared on Diagnosis: Murder after his series went off the air.
Doogie Howser, M.D. was still airing new episodes when that Roseanne episode aired.
Dang it. Sorry for not catching it before posting.
DM had some themed cameos that were fun- one had a few MASH alums, another had Wally and Beaver.
Dharma & Greg appeared as prospective purchasers of Charlie Harper’s house in the first episode of TAAHM without Sheen.
The characters of Martin and Ida Morgenstern from Mary Tyler Moore showed up on the MTM spinoff Rhoda. (The two series overlapped in time but AFAICT the appearances of Rhoda Morgenstern and her parents did not, so I’m thinking that counts?)
The final episode of Fresh Prince of Bel-Air had the Banks family selling their house. Some of the prospective buyers were George and Weezie Jefferson, and Phillip and Arnold Drummond.
In addition to Sam, Woody, Norm, Cliff, and Lillith, both Diane and Carla appeared on ***Frasier ***as well.
About the only members of the main cast of ***Cheers! ***who didn’t were Rebecca and Coach (who had, sadly, passed away years before).
If we’re talking about characters, we can include Frederick Crane, even though he was played by four different actors.
I thought it was the character or was it the actor, crossover?
actually mannix passed away in the second appearance …… and Andy Griffith was just sad to see