I just went through this myself, and I found a few posts that specifically said use 75W-140 synthetic for the Rubicon Dana 44's w/ lockers.
So rather than possibly damaging my Jeep, I decided to just accept the extra cost and put in the synthetic fluid. Seems to be running good so far (changed it on Sunday though, so it's only been in there for 2 days).
If you want sources I can point you to a few articles / posts regarding the fluids.
I had to pull the front cover off because it was leaking (or I guess "seeping"). I quickly learned why:
Didn't snap a "before" picture of the cover, but here's the diff itself:
The cover was really rusty at the bottom there as well. Thankfully most of the rust in the picture was on the old RTV liquid gasket, so the main case wasn't too bad.
After some cleanup:
The cover, after cleanup, you can still see some pitted areas where there was rust:
I've heard that some people will spray brake clean in...to be honest I didn't, just used a wire brush and some sandpaper to clean up the rusted areas that would seat up against the cover.
The back diff didn't have any issues, so I was lazy and didn't even pull the cover. Just drained the old fluid from the drain hole (on the right side of the diff at the bottom, when you're looking at the cover / fill hole) and put in the new stuff.
Black Aura - thanks for the info - I'm a service writer at a German car repair shop, so I know how critical fluid/processes are - fairly new to the Jeep world - we worked on land rovers for quite awhile - i'm kinda into using factory products/oem parts as much as possible, even though i know most of the stuff is quite generic anyways - im probably going to use a BG product/ their diff fluid (75/140) - its what bmw dealerships put in the 3 and 5 series M cars - although what i really hear is it doesnt make too much a difference..... i just want my **** to last - from what the maintenance records look like, my rubicon has 53k and no diff fluid change - doesnt seem to be offroaded though - no mud/etc inside the framerails - im pretty sure it wasnt much more than a daily driver before i bought it last week
No problem, glad I could help! Congratulations on the purchase! I'm also a fairly new owner...I've only had my Rubicon for a month now, but I'm loving it so far. On that note, you may want to check into more information, or let other people chime in on this thread as well, I'm just going by the information I've researched myself (and I'm fairly new to this).
On the break-clean thing, I have heard of other people who will spray a lot in there, but I never did find out if that was only for the stock diffs. I was worried it may damage the lockers on the Rubicon, so I decided to just change it out.
I just buy the 75W140 synthetic so that it's the same, but the live axles in our Jeeps are not too picky so the fluid choice is not that critical as long as you use quality stuff and keep the junk/water out.
I only use brake cleaner if I'm taking stuff apart (I like to stay clean...) or if it's really nasty inside (only happens when i work on other peoples rigs ).
BTW Dirt, the I'd bet the diference in spec between the front and rear have to do with loads and forces on the gears. Every time the Jeep moves the rear diff is under stress. 95% of the time you're in 2WD so the front simply spins freely so it needs less protection. The service interval on the front and rear is the same even though the front fluid should last much longer. My rear fluid always smells/looks worse than the front fluid when I swap.
-Darkstar, judy-judy. I'm going in for guns.
-Stick, splash one!
Hmm. In the 2005 FSM, it says 80w90 or 75W140 Synthetic, if towing, for Rubicon D44s for the front. For the rear Rubi D44, is only says 75W140 Synthetic.
The front diff's viscosity doesn't need to change for towing because the front axle is not engaged when towing. The Rubicon's rear diff's recommended viscosity is 80W-90 unless you are towing, then the recommended viscosity becomes 75W-140.