2002 Chevy Trailblazer LTZ 4x4 183,000 miles.
about 3 or 4 weeks ago it started making a grinding noise coming from right around where the diff. is. it starts making noise around 30 mph and gets louder with engine speed. sounds very much like metal grinding against metal.
Where do I start? how do I troubleshoot this? has new in the last 4 months caliper, rotors, pads and hub and wheel bearing on left side only. never worked on 4x4'* before but I understand axles.
appreciate any advice.

also, this started when I shifted into 4 wheel low just to get parts moving. when I shift it back to 2 hi, it makes a very loud bang and goes back to 2 hi.

Does the grinding noise go away in 2hi?

no. its there in every setting.

Always there just louder with speed? Are we sure the wheel bearings are good?

es always there and it gets louder the faster I go. doesn't matter whether I'm in 2 hi, 4 hi or 4 lo. I even shifted from 1st to 2nd to drive etc. still the same.
I put a new caliper and hub on recently and wheel spun freely but that was me doing it with it jacked up. seemed to be good.
I think my next step will be to remove tires and front axles and see what I can find on the differential. unless someone has better advice. I've never dealt with 4wd before.
I changed the fluid out on the diff and transfer case this past summer. I may drain the diff. and see if there are metal shavings in it.
I'll let you know how it goes but first I have to put a fuel pump in it. it'* supposed to be here tomorrow. that'* in another thread.
Again, any suggestions or things I should do different let me know.
and I'd really like to know who thought it was a good idea to put an axle shaft through the oil pan!
Thanks

The proper test for this would be:

1. Open road without traffic etc.
2. Make sure parking brake is off
3. Select 2-HI and [D]
4. Accelerate normally until the noise is as loud as described here
5. Let off the gas
6. Select [N]
7. Does the sound go away, stay the same, or drop with the engine RPMs?
8. Coast to nearly a complete stop and note how the sound changes
9. Stop the car
10. Select [D]
11. Go home
12. Report back in this thread what the results are.

Check all of this work, just in case something was missed etc. You'd sure feel silly finding it here after chasing the differential for a while.

Check that recent repairs aren't the cause as mentioned above.

This is a good Step 3

Personally, if it runs, I'd diagnose and repair this issue first. It shouldn't be driven much with this problem.

The engineers did because they didn't want to repeat the rollover scandal that the Explorer had been accused of by raising the center of gravity to accommodate the axle shaft living below the oil pan . . . and they didn't want to reduce the oil pan capacity such that oil changes would have been required every 1,000 miles. This made the beancounters very happy.

Disclaimer: My summary of decisions made at GM about this subject, based only in assumption.

already did the test about 3 weeks ago. 2 hi in Drive. accelerate to 80mph, noise gets louder with speed. put in neutral and noise doesn't change. stays matched with speed I'm going. coast to a stop as noise get less and less as I slow down. under about 20mph noise goes away. One thing I noticed. it doesn't seem to be getting any louder over the weeks. It also doesn't seem to have a drag on the speed I'm going, I let off the gas and it just coasts along just like it used to do, with the added noise of course.
I'm definitely going to check my hub, caliper... etc. especially after having a bad fuel pump that'* only months old.
I'll take your advice and do this work first and then the fuel pump. good advice.
having worked for GM, I know they always do things for a reason. Darn engineers.

Yeah, we love to hate them, and hate to love them, but as bad as a particular design might be, it could be worse. In the meantime, good ol' competition keeps them above some kind of average, otherwise the beancounters start saying things.

tore down everything to get to differential. this is what I found.
caliper seemed to tight to rotor. I couldn't even turn it with 2 hands. but it was like that last time I took it apart to put new caliper on, I thought that might be the problem. the pads and the rotor look fine though. took suspension off etc. took out cv shaft and it looked like it was fairly new.
in photo, this was in the transfer case. I think it'* a bad bearing, arrow pointing to it.
other arrow points to pinion gear shaft which has a mark on it where something is rubbing against it. I thinks it'* from the cv axle end. this isn't normal is it? does anyone know the name of the bearing with the scratch in it.
Thanks. Have to decide if I want to order new parts or get diff. at junkyard.