I was at the lake the other day and it was pretty muddy. I put the truck in 4wd and began reversing while the wheels were turned all the way to one side. I got that familiar knocking sound coming from the front underside. I have heard this sound before on many other 4x4 's . Does anybody know what causes this. the truck was in 4-high. Thanks.

1999 Ford F150 4X4
5.4 V8 Ex-Cab