lock up/ clunk after backing up

[krispycritters]

Ok so I have a 2015 gmc sierra 1500. after backing up and putting it into drive I get going for about 5 to 10 seconds ill get a jarring lock up for a split second. First time it happened I was going about 40 and I bounced off the steering wheel and left a little skid mark. It sounds like either my transfer case or transmission is locking up but it literally lasts for a split second then goes away. also I've noticed that after shifting from reverse I hear a loud transmission whine before it does it. This has happened at least 30 times for about a year and dealer says they can't recreate it.

It's the 6spd tranny

 

Also I know all about the normal GMC transmission clunk when shifting I have that too but I've learned to live with it. This is something else entirely. Its dangerous and I can't find anything on any forum mentioning this issue

Edited April 26, 2021 by krispycritters
additional info

[Snowcamo]

Trans pump on its way out.