I have a 2002 chevy suburban LT 5.3l v8 4x4. I recently grenaded my rear axle. Replaced rear axle and found out I also have a transmission problem. R works just fine. D&3 both wont work off a dead stop. 2nd works just fine like normal. 1st shifts rough on engage and runs fine until you accelerate more than 25% throttle then trans slips(goes full neutral till throttle is reduced or shifted into second gear). Also on that note once above speeds where first gear would be engaged in D&3rd gear both gears will engage and work just fine until below 15mph ish.

Just wondering if anyone has any suggestions for me before i buy a new transmission. On another note I've put approximately 200 miles on it like this.

 

Hate to say it but your tranny is trashed. when r and 3rd go out its a done deal. Way too many issues to track down and try and solve with only one fix. There are pressure problems, solenoid problems, clutch servo problems etc.