i seem to remember a tsb on a simmular condition. dont remember the number
or what the fix was....been a while since i worked in the shops.
hope this is atleast some help.

I once had a 1990 Dakota V6 w/auto.  Bought it used with over 100,000.
The previous owner said that the trannnie had been rebuilt one time.
The trannie was giving me trouble including not shifting into the 4sp
overdrive.  Several trannie shops said it need to be rebuilt.  

As a first move, I took the trannie pan off and changed the filter,
cleaned the pan and refilled the fluid.  It was OK for a while and
then once again it stopped shifting into overdrive.  Once again I
changed the filtter and cleaned the pan.  Each time there was a lot of
goop in the pan.

A couple of more times I repeated this action.  By the time I sold the
truck at 200,000 miles the trannie was shifting OK and was normal
except for being noiser than it should have been.

Old_Timer

AAMCO

beekeep