[DAWG]

could be many things on that truck
but since you have fwd then that rules out the pump.
When you checked the fluid was it a Brown color and had a burnt smell?
could be the pressure regulator?
the clip can fall out and the regulator assy will be sitting in your pan.
best bet is to pull the pan and change filter and look for anything out of the ordinary.
if you were to drop the pan you'd have some fibrous material in it. looks like rebuilt time!
you could have a stuck reverse valve in the valve body aswell.
TV cable isn't broke? that will cause a few problems right there.
lets face it before you came along that truck was sitting for years.
Working under a truck in the middle of winter is no fun.
You might have to take it to a tranny shop for proper diagnosis.
I had a couple 700R4s at my brother in laws shop, Ill see if he still has them.