Strange behavior from 4L60E

it may have the wrong clutch pak applied due to damage internally. If you got it from a junkyard get another one

It didn't come from a junk yard but the truck it came from was in an accident and the owner was parting it out. I personally went to his house and witnessed the truck go in every gear including reverse.

then check harness at transmission see if there is any damage there and scan for codes. does it do this while wheels in air or on the ground? Is the transmission full of fluid? What year did the transmission come from if it was from a pre 1995? If so then it is an OBD1 and will not work with your vehicle.

With wheels in air. It is full of fluid and truck was a 1999 Chevy blazer

What does it do when it is on the ground? The same? Tt may be just because the wheels are in air, but I kind of doubt it. But try it. If it does the same then it has either not enough pressure for reverse or a valve body problem and a transmission guy should look at it. Have you scanned for codes?

No but I'm pretty sure it's junk I just test drove it and it doesn't want to shift at all it will but only after you floor it and then let up. Still no reverse I have the worst luck. Know anyone who wants a 2000 gmc jimmy with 2 bad transmissions? Lol cause that's all I can do is laugh.