Ok...If it feels fine in man low and 2 that further reinforces my suspicions your forward and 3-4 clutch packs are likely worn to the point they can no longer hold against a normal apply circuit charge, hence the slippage...No 3-4 upshift can be a myriad of things but the most likely culprit is a 3-4 pack that is slipping under load to the extent 4th apply oil isn't making it to the servo...You may also have a worn 4th apply piston sealing ring in the servo cover or worn/torn 4th accumulator piston seal.Thanks for replying! The truck has 240k, I know it's been rebuilt twice already and the last rebuild was about 40k miles ago. The workmanship of the last shop was not the best I can tell you that but I haven't had any problems until now. I can shift it into manual low and it shifts fine from a start and into second and third but stays in third and never goes into fourth. When I have it in drive, it will start off slow like it's slipping
Was the converter replaced at the last overhaul?
What did you observe about the workmanship that leads you to believe corners may have been cut or it was not up to par?