1996 Chevy K1500

1996 Chevy K1500 5.7 vortec with the 4L60E. Tranny goes through 1st and 2nd good, hits 3rd and slips like a neutral. Can rev up and let off and get it to shift to 4th and then it will not go into overdrive which I assume is because of 3rd. I have already replaced the tranny with one I bought from a friend outve a wrecked pickup that I know for a fact was good before the wreck. Replaces neutral safety switch, ecm/tcm and still have the exact same issue as the first tranny. Any help would be greatly appreciated. I have tested all shift solenoids and they all work.

Test your output speed sensor.