I've got a 1986 CUCV with TH400. The odometer says 41k, but it might be wrong. The service history is unknown. The fluid on the dipstick looks decent to my eye (fairly red, no chunks). I've been reading some bad stuff about not changing the fluid and then changing it. I've also been reading some real bad stuff about tranny flushing. At the same time, you get less than half the fluid out if you don't flush...
), just let it be. Putting in new fluid will wash away all of the varnish that's holding everything together.
I have not had any problems with doing this on any of them..