Overdrive on 1999 GMC Jimmy

I own a Jimmy and it is always slipping into overdrive (revs up to 3 on gage ) then it quits and drives normal- it does this alot , what is the issue - is it a speed or overdrive sensor - also gas gage was working fine and now its stuck on E with the low fuel light on - just filled it up so I know there is plenty of gas .

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1 answer
You could be seeing the first signs of the transmission wearing out in your 1999 GMC Jimmy. It could be that a slight fluid leak in the forth gear apply circuit is causing 4th gear to slip on apply. I may be a good idea to have the powertrain control module scanned for fault to see if any transmission related fault codes are stored in the memory.