I have a 2000 F-150 4X4, 5.4 V8, with over 250,000 miles on it. There is a popping, clanking noise that seems to be coming from the rear end. There is also gear oil leaking out on the back right side of the truck. The noise is not at any certain point it is continous from the time you start to move until you stop.
First start from the driveshaft back move it around to make sure the universals are ok. Then move to the rear end. Have the rear up off the ground on jack stands or a lift and with the shifter in neutral and engine not running turn one wheel then the other. If you hear a clanking or grinding sound the rear would have to be pulled. You may have a broken tooth. Also check the axle that is leaking making sure the bearing is good. That could also be the cause for the grinding sound.