Jeep Cherokee Forum - Rear End Noise - But it's quieter when cold?

Here's one that I couldn't find exactly like searching the threads.

When it's warm out (NOT hot - above 60f maybe) and after driving for a bit, there's a terrible groaning/grinding noise coming from the back of my Jeep.

(Just bought it, 176,000+ miles)

Searching the board, I was going to jack it up and check the rear wheel bearings. Already checked the diff oil - my gawd it was RANK. Very dark, seemed perhaps a bit aerated (tiny bubbles....) not white tho, so didn't think it had a lot of water in it. Fluid was right at the bottom of the rubber fill plug so left it alone and figured maybe I'd buy a siphon and suck it out and refill this weekend. (Yeah, it's rank, and I know new gear oil stinks too.... but figured it may have never been done.) I did yank on the u-joints a bit and didn't notice a lot of play.

So it's been making all this horrible noise when turning and giving it gas. I thought it pretty much quieted out when I straightened out and at speed (highway, whatever) it's quiet. But driving around a neighborhood and making turns from stops it's horrible. Noise kinda makes me think the rusted up u-bolts holding the springs to the axle are loose and it's all shifting around! That's what it sounds like....

Well, this morning - the trip into work was cooler than it's been. (Upper 30's) All was quiet back there! No noise. WTF? It's a bit warmer now (mid-50's?) and on the way home, no noise. (I only live 10 miles from work, so it didn't get a chance to really warm up.)

So now I'm wondering what to do to it this weekend? If I get a chance, I'll drive it more and see if it is temperature related. I'm also sure I'll still jack it up and see how much play are in the rear wheels per other thread suggestions.

Has anyone heard of something like this? Maybe the gear oil is so toasted that when it gets hot and thin bad things happen? Or temp isn't an issue? It's our second car, but wife and I both work now so I got it as a "cheepie 4x4) for me to get back and forth to work this winter. I've already blown an additional $300 since purchase getting the shocks replaced and it to pass state inspection. (Front sway bar connecting links were shot) and really can't afford to have to run out and buy a new rear axle from a junk yard right now. (But, if I have no choice, better now then when there's 3 feet of the white stuff on the ground) :cry:

Suggestions? Ideas?

Oh, and here's the rear end info from the build sheet I requested from Jeep. (As I had no idea what I bought.)
4-Speed Automatic AW4 Transmission
Lock-Up Torque Converter
Command-Trac Part Time 4WD System
3.55 Rear Axle Ratio
Corporate 8.25 Rear Axle
Anti-Spin Differential Rear Axle

This is my first used 4x4. (All others bought new and under warranty when I had 'em) so I'm a bit at a loss as to what to check. Although I will say this - I like driving this Cherokee - much more than I thought I would. It FEELS like a REAL truck. The straight 6 is awesome - I've had V-6's, but this inline is a blast. I doubt I'll ever really go off roading (unless camping or something) but can't wait to get to work when there's a fresh layer of snow. :tt2:

Thanks for any help!