This weekend I was out in some mud and needed to put it in 4 wheel drive. I put it into 4-high on the go and noticed an immediate knocking coming from under the floorboard near the driver side front wheel. When I take it out of 4x4 the knocking continues for a few seconds but then stops. (I didn't try 4-low).

I suspect that this may be the cv joint. I Inspected the boots and there were no tears. The truck does appear to be actually engaged in 4 x 4 however when in the 4-high setting. The knocking seems to be in sequence with the wheels turning.

Any ideas?

Thanks

1999 F150 5.4 V8 4x4
153k
285/70/17 BFG A/T