Looks like the front pump died
#4 on my list.....
The reason I suggested over filling it is ..if the filter did fall off, it would have raised the level enough to get the truck moving again....
seeing as it didn't move under it's own power very well....would lead me to believe there was no pressure being built by the pump,,,or there was a massive internal leak...and the clutch packs cannot fully engage..
T-400's are pretty simple to rebuild, and with a few tricks can be built to withstand a lot of horsepower..there are a lot of free tricks and rebuild suggestions on the web....
A good trans shop may have a few stronger parts laying around from scrap transmissions....and if the rebuilder is worth a damn, can build it a little stronger for you for very little cost...
Cutting the clutch pack pistons down to accept another steel and clutch disk is very easy to do.
As well as using better sprag packages and using bronze thrust washers in place of the nylon ones.
heck,,,,even a trip to the local junkyard may help....If you can find a T-475
out of a delivery truck or old motor home, I would rebuild that instead...
the t-475 have straight cut gears in them and larger sprags...
check out this link...and good luck !!!!
http://www.fourwheeler.com/techarti..._400_rebuilding_tips_and_tricks/photo_01.html