86 300 sdl's tranny has problems...

At any altitude above 3000 feet my car's transmission shifts roughly, sometimes refuses to get out of second and when it does, will not go into fourth. I discovered this yesterday while on a trip up to Lake Tahoe- did not make for a pleasant experience as one might imagine. Is there a Barometric switch anywhere in the system that might have gone erk, or is there some other problem that might be the culprit? The Trans shifts normally here in the valley, if that's a help.

1986 300 sdl,"RennPanzer", 284K miles