I just had my front differential rebuilt on my Vitara.  I used a steel GV housing this time, a rebuild kit from Low Range and had a mechanic who specializes in rebuilds (but has never done a suzuki diff) rebuild it for me.  After installing it I took it for a test drive and everything seems fine except I notice a whine when I let off the gas (sounds like a manual transmission in reverse but not as loud).  Is this normal/acceptable or does it need to be set up again? It only happens when I engage 4x4. Thanks.

Do you have the one-piece hubs on your Vitara?

No I replaced those with manual hubs. 

Just to verify, does it make the same noise when in 2wd and the hubs locked in?

No it does not make that noise in 2x4 with the hubs locked, everything sounds fine.  Only when I shift the t case into 4x4. Which makes me think it is the pinion...

Also I realized the noise gets louder as the gear oil heats up, so the first minute you dont hear much then gets louder from there. 

I guess by now you have recognized that the whine is not normal?

Well i was hoping it might be normal for a rebuilt diff sinc everything is tight in there...from what I've read online it could it could cause failure or premature bearing wear OR it may just be an annoyance. I'm going to call the mechanic who built it today but I was hoping to educate myself ahead of time.

I called up the mechanic. At first he said it must be the pinion because he noted there were some small chips out of it while installing and there is probably nothing that can be done (but he did not see them as a problem at the time, just noted and mentioned to me). But then i asked why it never made a noise before this?  Afterall, it is the same pinion just a new housing and bearing right? So then he mentioned possibly adding another shim and wants me to bring the truck in to have a look see.. 

Even if the ring & pinion set up looks good it is hard to get it put back in the same place , its a shooting match to ever get it perfect and not make some noise. Thats why a lot of mechanics dont like puting in used parts .

Even if the ring & pinion set up looks good it is hard to get it put back in the same place , its a shooting match to ever get it perfect and not make some noise. Thats why a lot of mechanics dont like puting in used parts .

But my main concern is not the noise.  I'm worried about premature wear and/or catastrophic failure.

Your gear lash is definatly the issue.
 ...Boxcar...

If you have chipped teeth I would replace things as soon as possible. If they are chipped then there are likely cracks throughout the gear.  Last thing you want to do is strip a gear when you need it.

Did he use the gears from your aluminum diff?

Yes he did use the gears from the old aluminum Xl7 diff. Suzuki wants almost $500 for just a crown gear so I asked his opinion and he thought it would be acceptable to use what I already had and that he would file down imperfections and not to worry about it.  That said, thanks for the tip about likely cracks in the gears...

It seemed obvious to both him and I that the damage was caused over a year ago when the snap ring came off inside the diff due to worn bearings and went through the gears a few times before I stopped and unlocked the hubs.  When I got home i was able to fish it out the drain hole with a magnet. I replaced the snap ring and added a nylon support bushing and it seemed fine for another year until I cracked the housing this spring. He's got my truck right now going for a test drive and he says he will try to fix the noise if he can. 

$500 seems really expensive so maybe I will have to keep my eyes peeled for an entire carrier used but in good cond. Theres nothing available locally. I wonder if there are any lockers for my front end that replace the crown (since I would like lockers front and rear when money and time allows) and then I would just need to find the pinion?

He's got my truck right now going for a test drive to hear for himself and he says he will try to adjust and fix if he can.  Doesn't sound too hopeful though I gotta say...