Yeah, if you have something else to drive, leave the truck at home.

 

Also it wont hurt a bit to overfill it, specifically to get something slung in the tubes.

And yea, can't spin the pinion with an impact. Has to be a pretty beefy drill.

edit: Yea that sounds like outer bearings. You did timken right? It seems too loud to be carrier bearings for the differential. With that said, while you have the axles back out and cover off, spin the pinion again and listen for both pinion bearings and both diff carrier bearings and make sure thy are silent or near silent.

 

I did Timken, yes. I agree, does seem too loud to be anything other than outer bearings, although as you'll see in the post below, I must have been wrong. I did spin the gears by hand with the cover off, but to be totally honest I thought I knew what the issue was and didn't really want to disconnect the driveshaft again (especially since I didn't have a lot of spare loctite laying around, and I don't have a drill that will really spin the pinion fast enough) so I did not spin the pinion much. I spun the driveshaft by hand, but obviously with it still connected it's hard to spin very fast. In retrospect I should have found a drill powerful enough and done it.

 

Ok, sorry about the delay (took a few days for me to find the time), but here's an update, if anyone has more input/wisdom to bestow.

I took the new bearings/seals out, and installed the even newer ones, this time after making sure they soaked for a long while in gear oil. I also got better quality, original design seals. The bearings that came out looked fine, but I am not a bearing expert, so I proceeded.
While I had the diff open, I removed and examined the 4 smaller gears (the 2 diff gears and 2 spider gears), which looked great. I tried spinning the diff gears by hand, without the spider gears, and didn't hear much (maybe a slight sound on one side- hard to tell); I also spun them with the spider gears, and there was definite noise, but I'm assuming that's just the noise of gears meshing with no fluid.
The gear oil RTV was an absolute beast to scrape off, as expected, so I'll be using spicer or lubelocker gaskets that don't require RTV from now on lol.

Got everything back together, used new gear oil to be safe, and the truck sounded EXACTLY the same. No change. At this point, regardless of how much it sounds like outer bearings, I'm assuming the issue must be one or both of the differential bearings, right? I realize it could still be the pinion bearings, but I would think the diff bearings are more likely considering the lack of noise/play/roughness in the pinion. Regardless, I'm assuming the work/disassembly required to change the diff bearings/races would prompt one to do the pinion at the same time.

So here's my question: what is your advice?
I can do a lot myself, and have done virtually all maintenance/repairs in the 6ish years I've owned it, but I have never tried to rebuild a differential. Reading the steps online sounds quite intimidating, particularly when it comes to gear patterns, setting backlash, preload, etc. I don't have access to a full shop, and (sadly) do not have a hydraulic press.

I talked to a transmission shop (recommended by someone I trust), and they said probably $1350-$1550ish to rebuild, unless any of the gears/axles need replacing (pretty confident they don't). Seems like a lot to pay for a 23yr old truck with ~190K miles, although it should be noted I rebuilt both heads (fixing the dropped seat issue) several years ago, meaning the engine probably has at least 100K miles left.

My third option (other than the nuclear option- getting a different daily driver) is getting a complete axle assembly from a junkyard and swapping it out. It's the Chrysler 9.25, 3.55, which is not uncommon, although my understanding is the 4WD Dakotas/Durangos have a few differences- namely the longer left axle. My local pick'n'pull charges about $280.
Does this seem like a bad idea, since it's nearly impossible to know what kind of wear and tear a junkyard axle has been through? And is swapping the whole axle assembly as straightforward as it appears?

Sorry for all the questions, but looking for some guidance, as I need to figure something out asap (currently driving an old Cherokee, which is actually a lot of fun, but also borrowed).

Thanks again for all the help! Oh, and videos of the gears spinning below, if interested.

 

 

As opposed to crankin' out serious money for a rebuild, complete replacement is the best option. See if you can find a lower mileage rear diff, and swap it out.

Keep in mind though, I just spent 2500 bucks on my 170K 25 year old truck..... (parts and labor, I am unable to do that work myself) for a front end rebuild. Used vehicle pricing has gotten stupid, so I would spend significantly more on something else, and then have to spend yet more, to fix ITS problems. As any used vehicle invariably has them.

 

Spiders sound normal.

At this stage I'd be pulling the carrier assembly and do a visual inspection if nothing else.

The bearing caps are torqued to 100 ft lbs on the 9.25 and 70 ft lbs on the 8.25.