Well I guess the 4runner is going to be sitting on jack stands with all 4 wheels off the pavement for a bit. I gotta pull the third and send it in for the to inspect and hopefully warranty. I don't have a covered garage so while the runner is up with rear end disassembled I guess just tape bags at each end of axle and saran wrap the diff opening to keep debris from getting in?

Can I stash the rear axles in the truck or should I keep them upright so I don't mess up the bearings and seals?

sounds like a bad rear diff to me, not terribly hard to find a used one hopefully.

I think you found the issue. I'd really check that 3rd member out to make sure your gears have a good mesh. Low Range isn't known for the best quality of work.

Mike

Awesome...better taking it to a driveline and gear shop in the area and biting the bullet cost wise for them to repair vs sending back for them to either repair/ send new third?

I know nothing about setting ring and pinions, black lash, etc...

Wow

That is bad video. Take it to a professional shop. They will tell you what is wrong and what parts you need. If you are lucky, maybe just few hundred dollar fix!

Pulled the third today. Drained fluid Sunday when I hit 500 mile break in...drove 60 miles and new fluid was grey and shiny today. The underside of teeth have a wierd pattern. Note the diagonal markings in the middle. Look at the shavings on the drain plug...


Calling @VernySanders ...

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You have a bad setup. the reason youre hearing humming is because a lot of material has been taken off of the ring gear. You can see in your picture at the top of the tooth there are straight lines, those are called mill marks. Those should NOT be gone after 500 miles. I've seen mill marks on gearsets with 200k on them still with a proper setup. Your gearset looks like its already got at least a couple hundred thousand miles based upon the way that the face of the tooth has no millmarks and its shiny, you can almost see where there was metal exchange between the pinion and ring gear by the way it looks which is why you've got noise. All of the material that came off of the ring gear is on the bottom of that magnet and in the fluid. If you purchased these gearsets brand new, i would request the company warranty them out because i can tell you right now, these are going to be difficult to get a proper setup now that they've been worn the way they have.

EDIT: This is caused usually by lack of lubrication, excessive runout ( warped case half or ring gear ), or bad pattern / excessive backlash. Whoever setup the diff messed it up. Your backside, or coast side of the ring gear looks far better than the drive side. its got millmarks down it from top to bottom and because of the way everything turns over your coast side pattern will always be on the top of the tooth that way.

7/19/20 UPDATE:
So I have pulled my 4.88 third and posted photos. Still had vibrations when I put it back in obviously. I am told Trail Gear sees nothing wrong with third based on photos. I have now pulled the 4.88 third out and dropped in an OEM 3.93(?) third. Something is still off. I definitely do not "feel" a vibration with the current third. However now at 60 to 70 mph there is still a groan. It is almost like a dump grah grah grah sound that pulses at about 1 second intervals. If I push above 70mph ( which is a chore with this gear ratio) the noise stops. If I drop into the upper 50's mph it stops. I did feel like it intensified in volume on a banked right hand turn ( causing a left sided weight transfer) transitioning from freeway A to freeway B.

It maybe worth noting that when I did my 500 mile change the gear fluid was silver. 60 miles into that new gear oil is when I pulled the third to take photos. That fluid was still silver. However pulled the third to do the 3.93 swap and it was clean with about 100(ish) miles on it, although some debri on plug.

In all honesty I still feel the options are trans output shaft? drive shaft ( angles vs ujoints)? or axles ( wheel bearings, seals).

Again my axle bearings and seals were done a year ago, they still seem fine and spin smooth with no noise when I pull them to do the diff swaps.

My drive shaft has no play when on the vehicle. I also pulled it again while doing the third swap. No catches or play in either end u joints.

I don't have a tool to accurately measure driveline angles so I can't comment on that.

From what I can see of the out put shaft with driveshaft removed the teeth all look fine...

With that said in all honesty with normal music levels, windows down, and traffic noise I wouldn't notice this at all with this 3.93 gear ratio. SO this may have been happening prior to re-gear and I just didn't pick up on it. And if I hadn't re-geared I'd likely be driving with no issue.

The 4.88 definitely intensify the groan and vibration. And I definitely want to use that gear ratio but need to solve this issue...

Thoughts?

7/22/20 Update:
I'm starting to think it's a driveline issue. I used my phone with a clinometer app ( I know super reliable way to measure) .

Trans output is 7.5 degrees
Drive shaft is 10.4 degrees
Pinion is 6.1 degrees

Trans angle 2.9
Pinion 4.3

I have not been successful at finding what the angles should be for runner specifically. All say you shouldn't be above 3 degrees for most vehicles though. So do these angles tell me I should start adjusting driveline angles? If so how? Long or shorter rear LCAs?

As I sat here typing I've begun to wonder. I have OME 891 ( hd springs) on the rear. I haven't added bumper and weight to it yet. Although all center hub to fender measure 22.7 to 23 inches at all 4 corners. But could the springs with no weight be enough to change driveline angles?