Engine/Transmission (1994 - 1998) 47re transmission issues

[Flinty]

This transmission will lock the TC in first gear right after take off without notice, nearly stalling the truck. To avoid a stall I put it in 1st gear and shift manually up to drive. It will then behave normally until I park it again. Spent a fair amount of time last night searching but couldn't find any symptoms that mirror mine. Fluid is good. The transmission did slip a little bit when I first got the truck a year ago but that has stopped and this new problem has surfaced. Any advice is much appreciated.

 

[Flinty]

It was the speed sensor. Speedometer still worked but was sluggish. Truck runs perfectly now.