My T100 4x4 automatic transmission slips out of overdrive while driving. on 1996 Toyota T100

I recently channged the fluid and filter to try to remedy. The problem went away for a short while, but has returned. The fluid has turned very dark. My local shop wants to rebuild it, but I am curious if there is something more I can try myself. It also has trouble shifting into park sometimes.

Asked by for the 1996 Toyota T100
2 answers
I would take it to a tranny shop and have them look at it
seek a fre diag and est from aamco then shop prices, this can be a large bill at the end