Okay, 1988 f150 AOD. I have a small leak somewhere. I was sure it was coming from the pan. At this point there was no slippage. I changed the filter and put on a new pan gasket. After that I still have a leak somewhere and NOW it's slipping in and out in D. Reverse seems okay still. This happened almost immediately after I did the work. I made sure my fluids were full even with the leak. No sign of burnt fluid. Since changing the filter and gasket I have only driven the truck around six times. The slippage was minor at first but the last two times it is pretty severe (the truck has sat for a few weeks the last two times I drove it). When it finally goes in D it will shift in soft and slips in and out of gear at any time put again when I put it in reverse it seems to shift fine. If I shift in L it will go in soft put I don't seem to have slippage (yet).

Any advice is welcome

 

If this was the first time you have changed the tranny fluid and filter and it had alot of miles, I would drop the pan and change it again. Why you ask? Putting fresh fluid and a filter in the first time would cause the tranny to pump more volume thruout causing more debris to break loose and circulate thru the tranny.
Besides, if you have a leak from the pan, you need to seal that up anyhow.
You should have an inch/LB torque wrench to tighten the pan bolts in a criss cross pattern, torquing it in steps.
One more thing, you can get get more of your fluid out if you break loose the tranny line going to the radiator while BRIEFLY running the engine to totally drain your torque convertor. (make sure you use a tube wrench)
One more thing, with the fluid drained, now is a good time to change your tailshaft seal.
Good luck!