Grinding noise resembling bearing 93 F350

Maybe someone has had this problem before and can help.
I have a 93 F350 4x4 with 7.5L engine. E40D transmission, 1356 BorgWarner transfer case. I have noticed a periodic grinding noise that I can not seem to lacate. The noise reminds me of a wheel bearing going bad.
I have changed wheel bearings, spindle bearings, pulled the manual locking hubs, pulled the transfer case and had it rebuilt by two transmission shops, and still have this problem. I have removed the hubcaps thinking that they could be rattling. I have driven in 4wheel high, low and 2wheel high, it makes no difference. I have hadthis in three different mechanics shops and no one can locate this problem.
I have tried listening to the drive train with truck in gear on a lift, but cannot duplicate the grinding sound on the rack. I have no problem having the transmission rebuilt if this is the problem, but after throwing a lot of money at the transfer case on the advise of two diferent mechanic shops, I would really like to know that rebuilding will solve the problem.
One other thing, when the grinding started, I noticed a slight shudder when comming to a stop or acelerating from a stop.
Oh, and have also changed torque converter.
If anyone can help, thank you.