in the episode of Henry Get's Discharge did the guy Who played henry want to leave M*A*S*H

yes; McLean Stevenson wanted to leave the show. He had become popular as a frequent guest; and guest host on THE TONIGHT SHOW on NBC (MASH was on CBS) so NBC offered him a long-term contract; so he decided to leave MASH. But then, both his series on NBC-"THE MCLEAN STEVENSON SHOW" and later "HELLO LARRY"-were cancelled, NBC cancelled his contract and he couldn't go back to MASH, since they had killed his character. (I don't think he wanted to go back; anyway).
I heard that he made a BIIIIGGG mistake by leaving it.

I did not know he was a Guest Host of The Tonight Show