"What could it be?"

Gravity

Actually, I've noticed at times that mine does that too. I've never tried to troubleshoot tho since it's intermittent.

Try pulling each wheel & make sure there isn't something going on with the brakes (stuck caliper, stone stuck between rotor & caliper, ...). Check your parking brake too.

Check your fluids: front & rear diffs, Tcase, Tranny.

Lube your U-joints.

Check tire pressure.

Those are the easy things. If it's something more complicated - Ida know.