Front grinding noise
 

Hello, I've been having a problem with this for some time (years). The best way to explain the grinding noise I think is to imagine, your front axle how it slightly rubs against the rubber seal next to your pumpkin. But instead the axle is rubbing against a metal sleeve gently and as you go faster (say 70 mph), it turns into a very audible whine. Not nearly as easy to hear at lower speeds.

I think the most important re-creation of the noise is simply to drive in 4wd full time. The noise immediately turns into a somewhat grinding noise and is very loud. As soon as you step on the gas it will grind, and immediately after you take your foot off, the sound goes away.
I'm not sure if the whine at high speeds is related to the 4wd grind. Maybe that is how it is suppose to sound on the highway. But I can't believe that it is suppose to have the 4wd front grind noise. I figured I will add in the first paragraph just in case it can clue in on a certain part.

I've just replaced my axle u joints and saw nothing in the entire assembly that would cause the noise. My wheel bearings were really smooth and made no noise whatsoever. Not too long ago I also drained the pumpkin and saw no damage and very tiny metal flakes in the old fluid. This leads me to assume that when the power is being sent to the front, the driveshaft or something connected to it is what is causing the problem. I just recently found out there is something called a "pinion bearing" and I am going to assume that is the most likely culprit. The driveshaft u joints seem perfectly fine and have no play, or red dust.

Does anyone have any idea based off the description if it is the pinion bearing, or something else? If you do believe it to be a pinion bearing, does anyone know where to find a write up to change it (hopefully with pictures). Or at least inspect it. I'm not stupid but I've never removed the driveshaft. Much less the thing it connects to.

If nobody has any idea I'm thinking I could remove the rear driveshaft, put the front up on jack stands, put it in 4wd and let it idle in drive while I inspect underneath. Hopefully I don't have to do that but if anyone can confirm that that is safe to do then I might just try that after I check the pinion bearing and it looks good (if doing so is easy).

Sorry for the 2 page essay, I'm trying to figure out how to work on a car with no help except for you guys, and trying to be as clear and precise as I can. Need money for college so it hurts to send the Jeep to a shop. Thanks for all your guy's help so far, you have saved me a large sum of money and created some new hobbies for me, working on cars and having an addiction to tools ^_^.

Might temporarily remove the front driveshaft and see if the noise goes away. If it does it's one of the u-joints in the driveshaft- the one at the transfer case end is known for making oddball noises. However since you have a grind and metal in the fluid, I'd plan on bearings.