I took a quick search of this forum and didn't see anything on this problem. The truck shifts fine empty, 1-2-3 and into overdrive no problem. I hooked onto the GN and and it does the 1-OD shift but if I hit the OD lockout to hold it in third it will buck between 3rd and od and back violently.It wants to stay in OD no matter how much weight is behind it. empty its fine. The truck has been off the road for almost 2 years and hasn't done it before. TPS or other problems???

 

I'd start with the TPS.

 

how do you adjust a TPS or do you??

 

The tps is mighty expensive, just to trouble shoot, might try this route.

https://www.dieseltruckresource.com/...ad.php?t=42003

 

I had the same problem after I had about a 3,000lb load in the back. Sometimes flooring it for a few seconds would clear it up for 50-100 miles, then back again. Enfuriatings especially since I asked the owner if it ever did that before it was purchased ("no").

Anway, Yanking the TPS off and cleaning the hell out of it with Electro-clean did the trick, no problems since for me. Best of luck to ya.