The Fresh Prince Of Bel Air and The Jeffersons have apparently crossed over twice. The first time was in an episode called Will Is From Mars in which Will and his grilfriend run into the Jeffersons at a couples counseling session (no tension in those meetings I'm sure).

The second crossover was in the final episode of The Fresh Prince and Uncle Phil was selling the family's Bel Air home. As the episode went along, many prospective buyers stopped by to check out the house including George and "Weezie" Jefferson. Apparently George and Louise were tired of their deluxe apartment in the sky. In the end, I believe George and Louise ended up buying the Banks Family's house.

The last episode was sort of a chance for The Fresh Prince to tip its hat to other black sitcoms of the past. Among the other folk who came to look at the house were Arnold and Mr. Drummond from Diff'rent Strokes. It was a very funny episode. It was also very surreal to see characters from these otherwise unrelated shows showing up in the same episode.