noise from front???

Hey guys, im new to this site. Hoping someone can give me some insight on my problem. I have a 2003 four door 4x4 with the v-6 in it. I have 130,000 mi on it. It sounds like a wheel bearing out on it. The typical rump,rump noise. It seems to be louder at the speeds between 25 and 45. You can actually feel it in the body of the truck. I have had it to three different shops and every one of them cant find it. They tell me to drive it till something breaks. I have been driving it for the past 15,000 mi this way and doesn't seem to be any worse than it started out. Weird thing is, you can lock it in four wheel drive, and the noise is gone. You put it in two wheel, its back. I have changed the diff. fluid in the front to Royal Purple and no difference. I know for a fact its not a wheel bearing. Any suggestions?

 

Thats strange, but welcome to TW lol
Thinking about my only suggestion is get it up in there, spin the wheels by hand then in 4wd. Maybe an axle has some sort of play in it?

 

The shop actually checked both axles. said they looked like new.

 

Sounds strange, how about your tires? Are they oversized?

 

I had 32-11-50s on it. Bought brand new 16" Dick Cepek wheels with Hankook tires. down sized to about normal. Forget the size exactly. But I rotated them and nothing helps. I know its not tire noise, because when you lock it in 4x4 it stops.