New Tahoe. New problems

[Newkon]

I'll take a shot in the dark. I have once fixed similar which turned out to be a missing return spring(after tranny rebuild on older GM350). The springs purpose is to push the starter gear forward after the engine starts. If is is missing/broken the started gear is free to reengage and be 'tinged' back off the flywheel.
Test this theory by seeing if you don't hear marbles in your exhaust while taking off from stop in reverse. Good Luck !

Thanks for the words of wisdom. Unfortunately, it is at the shop right now. I pretty much had to go on their word because otherwise i would have been throwing money away. I had them open the tranny while it was out and they said that some of the clutches were worn out. Now.....I had no issues with the truck slipping or slamming into gear so it may be minor wear. They quoted me at $2400 for the rebuild which seems pretty high compared to what most people are saying that they pay for a rebuild. He said he will replace everything though. All clutches and all bands. One of the bands was rubbing on the housing that it folds around..What do you guys think?