I have a 2005 duramax the rear differential is locking up I'm having to put the truck in reverse to get it to unlock the truck only has 56k on it dealer says changing the fluid may fix the problem anyone with insight on the problem

 

When does it do this? Just driving unloaded in a straight line? or turning, loaded, hard burn outs?

 

Semms to do it when I'm backing up and then driving forward turning to the left I then stopand put it back into reverse it unlocks and then I'm able to go