1993 SUBURBAN DOWN SHIFTS, BUT WONT ALSWAYS GO BACK

Since you say the transmission downshifts, I assume it's an automatic? A possibility might be the vacuum modulator valve which is a little can like thing that screws into the transmission. It's an easy do-it-yourself replacement. You should also check the vacuum line for leaks.

This is probably not the problem, if your transmission shop knows anything at all. It should have been their starting point. If it is the problem, you've got it made. It's cheaper to replace than test, by the way.