It's slow shifting into 4th around 50-55mph and then after 70 it randomly downshifts.

No other problem in any gears.. Any suggestions?

 

I am not sure what could be the problem.. I know our Throttle Position Censor(TPS) goes out pretty easy. I put a potentiometer on it and that fixed my overdrive. Not sure on why your downshifting. After the Pot-meter I don't downshift at all.

 

I have been having an issue with my 92. The speedo and od quit working at the same time. I had the tranny pulled and went through...replaced drum and reverse band. Replaced the speed sensor on the transfer case, got the speedo back but no od. I then bought a potentiometerr, installed that and still no od...I've checked all the wiring over and over every appears to be fine. Any other suggestions to look for?

 

My odometer never worked when I bought my truck. The speedo after almost 2 years of acting up has finally completely died on me.. I have a brand new transmission and a pot-meter. Only things I can think of is wiring or plastic pieces that turn the odometer and stuff has wore down and cant grab. When my speedo first started to act up a year ago i would just push my trip meter reset and it would start working again.. I will have to look at my speed censor if that worked for you Marv. I've read a couple posts about plastic pieces wearing down. Sadly I'm still learning things on my diesel or else I would have something better to add.. My only overdrive issue was the TPS going out.