Is it the transmission? Here goes...?

"96" Chevy Z-71 pick up truck, 5.0 motor, 4 wheel drive. Had the engine and trany re-built two years ago. Brought the truck back after the slamming started. It didn't happen all the time and of course "Never" when we had it in to address the issue. It's not my truck. Today, I used it and coming home tonight I realized it only does it at 10 MPH and 98% of the time. Not every time when I make a left turn. What are your thoughts on this slamming? If you race it, it slams hard. If you gently excel, it gently slams. Is it first gear and is it a clutch? Is it other than the trany? We've asked many mechanics. They can't say for sure. I'll make certain it goes back to the transmission place and give them all I found out. Just looking for ammunition when I walk in there.