After two years, my son & I finally got his 86 Lariat road worthy….so we thought. The truck runs smooth up into third gear but will not shift into fourth & has a hard back pull when you lift off the gas in third. Only transmission work has been a drain & replacement of old fluid, new filter, & new transmission sensor. Automatic transmission.

Any thoughts?

 

I am assuming you have the AOD transmission and I would look closely at the TV linkage between the throttle body and the transmission for a binding condition or worn out bushings. Sometimes, the linkage will pop off the throttle body from a worn out bushing/clip.. I doubt the linkage is out of adjustment unless someone messed with it who is unfamiliar. Otherwise you are probably looking at an overhaul job on your very old transmission.