I agree, it's probably the front shaft or axle u-joints.

And BTW, I've gone 60-70 in 4wd... Last time was on the highway in a snow storm. Normally 2 lanes they only were plowing in the center of the two lanes. The big trucks going down the highway wanted to go FAST.

 

I do 65-70 in my 96 and 06 f250s with no issues. Granted the 96 has all new ujoints in the front axles and both drive shafts...

 

Nobody said it can't be done... The Owner's Manual clearly states you SHOULDN'T.

 

Eh, tomato, tamatoe