how many miles on the auto?
whats the ATF fluid look like?
IIRC the transfer case uses all its own fluid and doesnt share the transmissions fluid.
check all of your fluid.
if all checks well.... not dirty and at correct levels.
then next check that trans pan filter.
i also have a 90 4runner v6 with the auto trans.
mine shifts perfect and smooth.
but from experience, I'd check that stuff first.
