Why won't my 83 custom deluxe shift while driving?
The transmission may be worn out by now, should go to a transmission shop for proper diagnosis.
If you're on a budget and handy, check the vacuum modulator and line to it, it's a simple enough fix that might save you a trip to a shop.
I don't think there is one on there KenF.
hmm I checked again, repair manual shows a vacuum modulator ...
Found one, I know its around the time they did a redesign. So I agree with KenF. the modulator valve or vacuum line you should check out.
Looking for a Used C/K 10 in your area?
CarGurus has 313 nationwide C/K 10 listings and the tools to find you a great deal.
Search Chevrolet C/K 10 Questions
Chevrolet C/K 10 Experts