Ok first off you should have a 30 front and 44rear.So, at 7500 miles, went ahead and swapped out the oil, and the diff fluid. I like Royal purple, and used it in all of my previous vehicles, so I'm keeping with that tactic on the JK.
A few questions though:
(I have dana 35 front, d44 + LSD rear, 3.73 gearing)
1) My front diff fluid was a light tan, milky color with lots of little metal bits in it.
-> I presume the milky color means I got water in it? am I right here?
-> I presuming the little metal bits are from the gears being new and this being the first fluid change on the front diff? I pulled off about half an inch of metallic goo from the magnetized drain cap. Should I drain it again soon and see if I still get lots of metal?
2) The rear diff fluid was black, blacker than night. I'd guess this is because it gets a lot more wear from being always powered? It was a lot blacker than I had expected - think dino engine oil that's been run for 10k miles! I think it had plenty of metal in it, but couldn't really tell, it was pretty black. Not nearly as much material on the magnet as on the front.
3) I put 75-140 (Royal Purple) in both diffs - I know the manual recommends 75-90, unless doing heavy towing. I had a bunch of 75-140 left over from the commander, so I used that. Any known issues?
And, finally, a comment. It runs a LOT smoother now! Drove about 35 miles after swapping everything out, and it really seems to take off smoother. I wasn't getting "chatter" before, but I could feel something rough in the drivetrain when accelerating, and now I don't feel it. No chatter going around corners, so I'm presuming I don't need to add any LSD modifier.
Milky oil means water in the oil
As for the metal shavings, if they persist then you should get it checked out. As for the oil being black that is normal.
75-140 should be fine. This is what they recommend for TJ's and the book calls for 80-90. Also do you need to add a additive in the rear for the LSD or not? Your manual should say so.
Hope this helps