Rumble in the rear
My E-150 did that last summer. When it got to the end of my driveway it went "CRUNCH, CRUNCH," as I was turning to get on the street. I backed up to the house and shut her down. When I got the cover off, I found several gear teeth in the housing. Mine has the 8.8 ten bolt posi trac. The side gears had given up so I pulled the third member and replaced all four gears, the frictions, steels, and bearings. That did the trick. If you get lucky like me, you might not need the ring and pinion. Those are complicated to get set up right the first time if you have never done one before. Anyway, the side gears in the third member are what makes the low rumble. Specially when your in a turn.