1991 Chevy K1500 automatic transmission problem

This is actually a friend's truck, but he had an underhood fire on the way to a jobsite pulling a small camper on a hundred degree day. The fire was near the firewall and behind the distributor. They got it out and he continued on his merry way (about 20 more miles). He noticed it wasn't shifting correctly and a test drive confirmed that the transmission is not shifting until very high rpms (the speedo don't work and has no tach..it's a fencing truck not alot actually works). the problem is not isolated to just the 1-2 shift, but all of them. I also noticed that it stumbles in gear at idle, but clears up if you put it into neutral. The TV cable seems to be working corecctly, but the tranny oil is burned and old (going to replace filter and oild this weekend. Any help here would be most appreciated. P.S. not that it matters much, but the engine is the TBI 4.3l. Thanks in advace!