2019 2500HD trans issue ( wheels spinning while trying to stop!)

[2019 Red]

I have a 2019 Silverado 2500 High Country that we mainly use to pull our 9000-pound travel trailer. 

It does a wonderful job doing towing and we love the truck. It has just over 67200 miles on it.

It has 6.6 Duramax diesel with the Allison transmission.

over the last month on three occasions, while not towing, when we made a stop, it has seemed the

transmission did not disengage. 

 I had to almost stand on the brakes to stop but the rear wheels kept slowly turning. The first time,

when I let off the brakes, the truck lunged forward. The other two times I have shifted to neutral

to release the transmission.

 My truck has been parked for a month waiting to get into the GMC service department where we

bought it February, 2022.