The A4LD that would be appropriate to your truck is nowhere near as reliable as the M5OD that should currently be in it. It sounds like your trans came out the losing end of a previous owner.
The real catch to older power train assemblies, engines, transmissions, transfer cases, etc, is that to throw money at repairing a single failure generally is about as pointless as every thread on this board eventually becomes. In the long run it is generally far cheaper and easier to replace the whole assembly with a remanufactured unit that comes with a warranty since once you take it apart to fix the current failure something else is bound to break. Stirrin' up the gremlins and feeding the mogwai after midnight and such.
Buy a trans from the bone yard or, if you plan to keep the truck for a long time yet, a reman unit, and fix it right.