CypressF150

I have a 2013 F 150 Ranch Off Road. I have seen may threads on the solenoid issues; its not that. Both have been replaced and vacuum test on the system performed; all passes. In 2-WD, as soon as the back tires spin, I get a grinding noise in the front end? Sounds like potentially transfer case clutches to me? The dealership took me in a new 2014 limited and it too had a very slight sound but nothing like mine. Over time, mine growling gavel sounds has become more noticeable. To give you in idea of sound, it sounds like a bad CV joint than has sand and gravel inside. No, the truck does not have a torn CV boot nor, has been in any deep mud or sand. I do take it off raod at the farm and pull hay trailers through 2-6" deep mud form time to time and I use the 4 x 4 to pull a 24 foot boat out of ramps.