98 sidekick 4 speed auto high rpm shift problem
 

i am trying to help my father-in-law with his 98 sidekick. it has the 1.8 and the 4 speed auto with about 90,000 miles. it has begun to shift at a higher rpm than it usd to. the overdrive cutout switch and the normal/power switchs both work properly and it is not stuck in the power mode. the transmission fluid was a little dark but not burnt or anything unusual. i changed the fluid and filter just to be sure and before i changed it i ran a can of flush/cleaner in it. the filter seemed clean and there was nothing unusual in the bottom of the pan or on the magnets. it still seems to shift at a higher rpm than normal.

is there any type of vacume modulator or something external that can be checked? i am not real familar with the internal workings of auto transmissions. is there something with the engine itself that could cause this problem? all help will be appreciated.[/align]