Has anyone ever seen something like this? A friend and I regeared my rubi-44 from 4.10 to 4.88 a bit over a year ago. We spent a solid 8 hours on each axle, shimming and reshimming until the backlash, preload, and contact pattern were dead center in the FSM ranges. I broke the gears in with several short trips around town, never reading more than 150°F on the case after. I stopped halfway through the first 150mi highway trip (a month later) to feel the case, and it was cool enough to leave a hand on for several seconds. Diff oil was changed after that first trip, ~400mi after the swap. Oil had a greyish tint to it, but no visible flakes present, and the drain magnet had a normal amount of fuzz on it.
Roughly 8 months after the regear, my inner pinion gear started whining. When we pulled the old one out, the rollers and inner race looked like new, and the outer race looked like it had a couple impressions in it, maybe .003" deep. Almost like a slightly out of round roller had pressed on the same spot repeatedly. That was weird, but we inspected the rest of the diff and found nothing out of the ordinary, so we replaced it. We re-set preload, backlash, and contact patch to FSM spec at that time.
Roughly 4 months after that (recently) my rear diff started making the same noise again. Thinking i had gotten a cheap bearing last time (used a "master setup kit" from a 3rd party supplier), I ordered boxed component bearings directly from a Timken supplier and made arrangements with the same friend to help change them out. When we pulled them this time, though, they all looked completely spotless. We could not see anything wrong with them, but the noise wasn't as bad this time so I thought maybe we caught them early enough not to be visibly damaged. We put the new ones on and reset preload/backlash. We had good numbers, but the gear mesh was making a worse noise than when the supposedly bad bearings were on. After messing with every variation of carrier/pinion offset, and running just carrier / just pinion tests to confirm that it was definitely the gear mesh making the noise, we tried to get the contact patch dialed in (thinking that if the contact patch was good, it might just need to be run under actual load to wear in).
We didn't have gear paint handy, but heard that thick oil based craft paint would work in a pinch. The first few times we tried to get a pattern on that, we thought it was failing because that paint wasn't working. Turns out the paint was fine, but the drive face of my ring gear was NOT TOUCHING the drive face of the pinion gear... Somehow, the drive face of the pinion (and a bit of the ring) had worn down to the point that the drive face of the pinion was only scraping over the knife edge of the drive face of the ring gear. We also noticed that by running the diff from a drill for 50 revs or so, that the drive side of the pinion picked up paint from the very edge of the ring teeth, and deposited it on the very edge of unpainted teeth, confirming that there was only edge-to-edge contact. The coast side of both gears is fine, so I don't believe that the housing/carrier/pinion shaft could have bent and changed the mesh depth/angle.
Closer inspection suggests a full-face contact patch on the ring at some point, and massive full-face wear on the pinion; it looks like almost .100" missing over most of the pinion drive side, and a clear ridge below the wear pattern. Can anyone offer an explanation for how this is possible?
-The diff never had water or mud in it; the oil was changed 2-3 times/yr and inspected for dirt/shavings.
-I do not tow with the Jeep, and usually draft an 18-wheeler on the highway at 55-65 when I need to make longer trips.
-The diff has never run hot or made strange noises other than the bearing whine mentioned earlier.
-The diff did not exhibit any rolling resistance (would creep on the slightest incline when in neutral at stoplights)
-The rear driveshaft is a double cardan, in good balance, with the back end pointed directly at the pinion shaft.
Pics of the strange wear below: