It sounds like the rear band is toast from not running the kick down linkage.
Tranny must be tore down to replace the rear band. It's not too hard.
Chances of a fluid cange and adjustment fixing it... probably very small.
Pull the pan and check the rear band, it'll probably be very loose.
In my bro's 727, the rear band wore out and the previous owner tried to readjust the rear band. Apparently there wasn't enough adjustment to tighten the band, so they replaced the adjuster bolt with a longer one. It didn't work, here's what happened to the rear drum:
Tranny must be tore down to replace the rear band. It's not too hard.
Chances of a fluid cange and adjustment fixing it... probably very small.
Pull the pan and check the rear band, it'll probably be very loose.
In my bro's 727, the rear band wore out and the previous owner tried to readjust the rear band. Apparently there wasn't enough adjustment to tighten the band, so they replaced the adjuster bolt with a longer one. It didn't work, here's what happened to the rear drum:
