Question 1 was the truck in 4WD in the recent past?
2nd has the auto hub assembly been serviced in the last 10-15K?
3rd is the truck in and out of 4WD often (as in once every few weeks even for only a few feet)?
If you answered yes to 1, no to 2 and 3 then you may have a hub sticking.
If you answered yes to 1 and 3 and no to 2 then you may have a hub sticking.
If you answered no to 1 and 3 and yes to 2 then you may have a problem that need checked.
If you answered no to all questions then you need them checked.
Grinding and thumping in 2wd is not normal even if the truck has been in 4wd and has been driven and backed up just a few feet.