There were 18 episodes and everyone had vacations from being away on episodes. Have you noticed that? I think Arnold, Mr. Drummond and Sam appeared more than anyone else. Heck I think Dudley appeared more than other main characters.

Yes, there were budget issues during the last season, which happens with a lot of long-running shows. The cast members tend to get large raises when a show becomes a hit, but they don't take pay cuts when the ratings decline, at which point the network won't increase the show's license fee. So, they have to come up with ways to cut the budget, and in the case of DS, they rarely had episodes with the entire cast.

Check out the later seasons of "Dallas" for an example of the same thing. A lot of long-time regulars were let go, and there were no longer scenes shot on location in Texas. During "Knots Landing's" last season, each cast member had to miss two episodes. The show was ultimately cancelled for budget reasons, because the ratings were still acceptable, but it became too expensive.

I know about "Knots Landing" Michelle Lee volunteered to do 4 episodes without pay. So the same did happen with Diff'rent Strokes.