Differential question.

My son's XJ has torn up a pinion bearing. He discovered this when the pinion jumped teeth on the ring gear. 

An inspection finds gray gear oil from metal in the oil. Inspection of the ring and pinion show no noticeable damage. No chipped teeth. No thinning of the teeth. No obvious wear. These operate without load 99% of the time. Basically in coast unless it is in 4WD. 

We will be putting a bearing kit in it. My question is, should we replace the gears even if there is no obvious damage to them? A set of gears is about $200 but we are spending my son's money sot that's a fair amount to him. 

What say ye?

You should set up the r&p regardless when you replace all of the bearings.

I personally would just swap in a used D30 from a Pick and Pull or off Marketplace.

If they look OK and set up OK, then I'd run them.

I think it depends on how and how often he uses four wheel drive.  If it's not often and mostly low speed then I'd be tempted to reuse them.

In reply to Russian Warship, Go Berkeley Yourself :

It doesn't have stock gearing. Otherwise I have a couple of spares we could use. 

I would set them up and full send.  They might be noisy, but they're not any weaker.  They're only used at slow speeds.

He likely has hubs that disengage, so they won't even be turning unless getting torque.

Keep all the shim stacks in the same spot and same amount, and you should be remarkably close.  Shims are there to account for differences in housing machining, not for differences in gears.  Since you're keeping the same gears and housing, should be a slam dunk.  I suppose you'll be able to tell how much wear they got when you check backlash on the stock shim packs.

In reply to Curtis73 (Forum Supporter) :

The XJ didn't come with unlocking hubs unfortunately so the assembly rotates all the time. Just with no load. 

We will get it apart and do a closer inspection. If there isn't any noticeable damage I reuse the gears. 

The XJ could have a FAD, depending on model. My XJ is a Sport so no FAD but if your son's is a higher trim package it may have an FAD.

In reply to buzzboy :

His is a Sport as well but the open diff was replaced with an Eaton Detroit Truetrac LSD several years ago when I regeared it. 

I'm wondering if I screwed something up when I set up the new gears. 

Russian Warship, Go Berkeley Yourself said:

You should set up the r&p regardless when you replace all of the bearings.

I would assume (having seen his posts along the way) that he does know that, and is asking it the gears could have damage not evident from a visual inspection. 
 He stated the bearing failed. 
I feel that the failed bearing is why the gears jumped; if the teeth look good, replace the bearing, check the setup, and send it. 
Even if ratio was the same, a JY rear has a higher chance of being worse , than it does being better!

Ugh... I have seen a front Jeep diff skip teeth exactly once, and it either caused or was the result of the diff carrier bending.  The ring gear ended up having many thousandths of an inch of runout.

This was on a four cylinder Wrangler.

This resulted in much, much carnage.  Is he sure that the front gears skipped and it wasn't something more simple like a jumping chain in the transfer case?