This is something I found while looking for a similar problem:
Q: i have a 4r70w in a 95 f150. first of all what type of fluid should i use. and secondly what would cause the truck not to get any acceleration. it works in reverse, but thats it. there is nothing in 1,2,3,or 4th gear. if someone could help that would be awesome. thank you.
A: I always recommend sticking with the factory specified fluid if the transmission is stock. For your truck it would be Mercon fluid. Based on your transmission's symptoms it sounds like you have a worn forward clutch. Worn forward clutches would cause a loss of 1st, 2nd, and 3rd gear. If you could accelerate the vehicle 4th gear would still work, so I am guessing you just can't get to fourth gear because you can't get the truck moving. The transmission sounds like it is going to need a rebuild, but if you wanted to be really sure you could have it tested using an external manual control box that connects to the transmission wiring and can command each gear without requiring the vehicle to be moving. If fourth gear (overdrive) works this way, then it is definitely your forward clutches. The problem has to be mechanical or hydraulic, as there is no electrical failure that could cause your exact problem.
