I have a freshly rebuilt 4L60E in my Tahoe, after starting the vehicle you can shift the vehicle the first and second and it moves perfect along with reverse and park is solid. If you try to go into drive or third gear it will not move just acts like a neutral. If you pull out fuse 20 the transmission fuse and put it in third and move solid and works perfect. If you go to Overdrive it tries to move but doesn't seem to have the power.

 

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Take it back to the rebuilder and have them repair your transmission.