I was backing up the other day and noticed about ten to fifteen feet in reverse (0-2mph) I felt a bump, like when shifting into drive. I thought I ran over something in the garage so thought nothing of it at the time (hopefully not the dog). But, now I notice it every time I back up anywhere, although it only comes from an angled turn, not dead straight, and only when warm. The fluid level is fine and it's got Redline synthetic 75w-90 in it with 4,000 on it and 14,500 on the truck. My thoughts are the locker, but why would it do that? Any thoughts?