+1 on a lockout hub. If it sounds like the axle, it's probably the hub. Auto hubs were one of Ford's "better" ideas lol... as soon as I get back on my feet money-wise, I'll be replacing mine with manual hubs.

Another possibility is your transfer case ate a shift fork. Mine did when the oil pump let go, and it trashed the shift forks so the transfer case wouldn't completely engage in 4H/4L. 2H worked fine for a while, no noise whatsoever but it would randomly go to neutral for a second and make a quiet ratcheting noise... then back to normal like nothing ever happened. Rebuilt a manual shift model, swapped it in, and been happy ever since.

I don't think transfer case is your problem though, mine made a grinding sound directly under the cab, unmistakable that it was trying to say HEY I'M YOUR TRANSFER CASE AND I WANTED YOU TO KNOW I'M AFU!!