I have an 09 H3t with a 6.2 LS In it with the 4l60hd transmission. I’m nit entirely sure what’s going on with the transmission so I figured I’d check the expertise of the forum. Wrote I get it in the shop.
the transmission has about 60k miles on it.

When you give it about 3/4 throttle in drive the transmission slips out of drive and then you have to manually shift it from 1st all the way back into drive. And then it runs fine In drive bit the second you give it about 3/4 throttle it slips out of drive again. But if you give it 3/4 throttle in 1st 2nd or 3rd there is no slippage.

im going to change the fluids this weekend. It just figures I’d check for some ideas on what could be happening. Thanks for any feedback.

 

while your are changing the fluid take a look at your shift linkage on the transmission. the bushing on the link might be going.