My '92 B250 has to warm up before it will shift into over drive. Sometimes 5 miles, sometimes 20 miles, sometimes I have to stop and start before it will shift. Anyone know whats wrong?

 

could be the solinoids in the trans pan attached to the valve body?
had that same problem with my 92 ramcharger at one time.i changed them out,and it was good as new. not saying that is your problem but it might be worth looking into.
good luck..

 

The van has not been driven much and sits for a year at a time. I think I will try an additive to the fluid. Only has 87,000 miles.

 

maybe try that seafoam (trans tune),for transmissions,i tryied it works.
i really felt a difference in the way my 93 d150 shifts, at times i get a bark from 1st to 2nd,depeneds on how much foot got on pedal.
good luck