It is rebuildable, if you have the skills you can get a Rebuild-kit for about $340, buy a New-Rebuilt one for about $1100 plus shipping(for rebuilt transmission and your core), pay a shop $1500+ to rebuild yours, or look for a used one in good condition on Craigslist, wrecking yard, etc., should be able to pick one up for under $250. If you go the used route, try to find one from an '89-95 Truck/4-Runner, or from a turbo, they tend to hold up better to high mileage than the earlier transmissions.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed