went to check rear diff gear oil other day and it was low.. humm... so I figured I'd top it off. fluid that came out was really grey so decided to drain and fill. There was soo much metal shavings stuck to the drain plug and the fluid was really really dirty/grey from metal. This was an e locker that I swapped in maybe a year ago and changed the gear oil after 500 miles. should it have that much metal in it? thanks!

Large chunks are no good, you've got a problem, hard to tell how bad it actually is without pulling the diff out to inspect the teeth and bearings.

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How are your wheel bearings? If they get a bit loose then the ABS sensors can grind against the sensor rings. Which is a different problem than the diff bearings eating themselves.

Is it making any noise?

Your diff probably had more miles than you were told. 200k is the lifespan.

both wheel bearings have been replaced with in the last year.

got it used from a gear and axle shop.. he measure the lash and said it was good. I'm sure it had alot of miles on it though.

no large chunks, just a lot of shavings. wish I took a picture when I did it.

Yeah he was just trying to sell you it.

Low fluid caused excessive wear. Where did that fluid go? Check brake shoes.

Anything bigger than "glitter" in the oil is a big chunk, depending on how long the diff sat there's a small possibility you can get more life out of it, if there was enough "glitter" mixed into the oil to noticeably change the color of the oil when light was shined through it, probably a bearing eating itself up and not gonna last long.

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I build diffs for a living, if it was me I'd pop out the 3rd member, grab a ton of brake clean and spray everything out and if you have compressed air then blow it out. (Bearings etc.) As long as everything turns over fine you're good. Honestly if you really dont want to drop the diff then change the fluid every month untill shes clean. Diffs are fed lubricant from the ring gear, if there was a lack of lubricant due to low fluid there was more than likely metal transfer from the ring and pinion. This would explain your shavings. As long as it's not running loud aka you can hear abnormal noises from rear end while driving then the metal transfer wasnt bad enough to cause any type of crazy damage or high spots causing knocks. The other thing that can happen is when you initially put new fluid in it, it could have cleaned up the inside of the diff which already had a bunch of metal floating around because the previous owner never changed their fluid. What messes up a ring gear and a pinion is when you change the mating teeth. If you never remove the ring gear from a carrier and reinstall it to the same pinion a diff will last untill the bearings go or the metal goes enough to the point where a tooth chips, the toyota OEM gears are heat treated like any other but also have thin teeth on the ring gear, this allows more of the total metal on the tooth to be hardened. If the guy you bought it from took the ring gear out to inspect everything then re installed it there are now new teeth wearing at abnormal rates due to never being meshed before. Once they ware and the teeth become reaclimated to one another then they're fine. There is no true mileage limit on gears, OEM gear sets cannot be put into a mileage box. After markets for sure can be though. Diffs are one of the simplest and and bulletproof components of a vehicle. If you have no noise on the diff dont even worry about it. Just my two cents.

guy I bought it from pulled it right out of the housing while I was there, so no new components. Now that I think about it the fluid that came out at that time was dirty as F. So maybe the PO of that rig never changed it and beat the F out of it. No sounds at all it's totally quiet. I'm just going to keep an eye on the level and probably change it out more often in hopes that cleans it up.

Guy I bought it from took some special paint and put it on the gears and looked at how the paint got rubbed off. and then used some tool to measure how much play was in the gear. (excuse me for not knowing the proper terms)

The other culprit of excessive wear due to lack of lubrication that comes to mind could be increased runout. Runout is caused my a warped case or flange surface or warpage of the ring gear. When it warps your backlash will have different readings all over the ring gear. The difference from highest to lowest backlash is your total runout. More than a .005 runout could be HUGE deal for these rear ends. In the ones I build our spec is .010 because our ring gear is 16 inches wide. I think max backlash on these 3rd members is .008 if you had even a .003 distortion due to heat then there would be uneven wear as well. As long as you keep the oil clean, I would still feel comfortable with running the diff. As long as next time you check it theres no shavings.

Thats exactly what I would do. He was checking the pattern with pattern paint to show that it was still hitting where it should on the tooth. If it wasnt hitting right you would need to or adjust your backlash to move the pattern on the tooth. You see guys on forums all over the internet with 37's or 40's talking about gearing back to the 4.10's or 4.30s because theyre tired of breaking gears, the OEM wont do ya wrong. good luck bro!