I have a 2001 F-150 with the 5-speed with 50K miles now. I just noticed today that when I put it in reverse, and first go to take off, I get sort of a grinding / squeeling noise that sounds like something is in a bind. It sounds as though it's coming from the rear / middle of the truck as opposed to the tranny. It only does it for like the first 6 feet, then goes away. I pulled the truck up a hill and played with letting it roll down under it's own weight and it didn't do it. I held the brake while attempting to reverse and it doesn't seem to do it. It's only on level ground when reversing for the first few feet. I have sort of ruled out the tranny. I'm thinking rear-end or U-Joint. What do you think? Thanks!!!

Note: with the e-brake set and tranny in gear, (parked) the drive shaft has quite a bit of rotational slack. You can hear the gears in the transfer case and rear-end.