Check the transmission shift detent cable before assuming the transmission is bad. I've seen 3 Gen I automatics recently with the trans cable jacket pulled out of the crimp. Slide the rubber sleeve back near where the cable ties into the throttle body linkage. When it slips out, the transmission will shift erratically, with typically very late, neck snapping upshifts. It's not fun to change it(drop the pan and remove the valve body), but a replacement is only about $50.

1987 Raider
1991 Montero LS
Nipomo, CA