Transmission problem? '05 TrailBlazer

It will shift normally from 1st to 2nd and 2nd to 3rd but then no further. It's like it can't go past 3rd gear. The check trans light is not on. The fluid is full. The engine seems fine but is straining due to being in 3rd gear. It has 130k. It had been shutting off when stopped at a redlight but that has stopped and now it just won't go into overdrive. What can it be??
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