Drive shaft is not turning in reverse.

I added more ATF and now I'm way over the full line, but no luck.

are you hearing grinding noises in the trans in reverse?

No. I checked closely today. No grinding noises. Reverse acts the same as Neutral.

The trans is a TH700R4.

This might be a bone-headed question, but:

The bottom is packed with snow and ice. Are there are linkages that could be getting blocked and keeping it from going into reverse?

.....or is the transmission self contained and nothing outside of it can cause this problem.

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.

Probably not, but if it's like a 4L60E, which I assume it is, the linkage is on the side of the trans, and I guess it's not inconceivable that a chunk of ice could get in the way. But if that were the case I believe the shifter would not go into reverse. I would still suspect an internal problem like a valve body.

Thanks Dawg and Formula!

Time to find a local transmission shop or a decent used TH700R4.

It looks like the problem is fairly common.

any update on the truck David?

No updates Dawg. The truck is still stuck stuck there.

My father-in-law suggested that I have AA Automatic Transmission in Canton CT tow it to their shop. As soon as some more snow melts, I have to take the plow off first.

AA Automatic Transmission web site: http://aa-automatictransmission.com/

Ive heard good things about that place from others .
a free tow to the shop is nice too!.