1995 Ford F150 5.8 4X4, with manual locing hubs. There seems to be no end of it on this truck.

Its been snowing in Utah. On my commute, I have around 10 miles of mountain driving, and 14 miles of freeway driving. My last 3 trucks have had manual locking hubs as well, so I have a system: I lock my hubs and use the 4x4 on the snowy, twisty roads (>30PMH), then I leave them locked and turn the 4x4 off for the freeway (70MPH)

Today was the first time I have used the 4X4 on this particular truck. It seemed to work smooth and quiet on the mountain roads, but as soon as I got to the free way and turned off the 4X4, with the hubs locked, I had a very loud, almost grinding "thrumming" noise that was both heard and felt. To test a bit, I threw it into neutral, still noisy, then I engaged the 4x4 again, and oddly, with the 4x4 engaged, it was quite except for a slight hum.

Whats going on here? Bad front differential?

 

I would check u-joints in drive shaft,jack it up and spin the wheels see if you have play in the axle shaft u-joints.It may be in the locking hubs