I recently just did this in my 200,000 mile truck upgrade post you can check it out. Heres what I did:
1) got a new cover (so i could paint and have preped plus with a better drain hole, hate the factory grommet one that always leaks)
2) got RTV Sealant
3) put shallow pan underneath pan start with bottom two bolts and work your way up using a screwdriver to pry old gasket off.

MAKE SURE TO COMPLETELY CLEAN THE LIP WHERE THE GASKET MEETS, I USED A WIRE BRUSH.

4) Take a bunch of paper towels and get as much gunk out of the bottom of the rear diff. I sprayed and wiped clean with brake cleaner trying to stay away from spider gears.
5) let dry and put a bead of RTV sealant on both sides of the gasket then stick one side onto new diff cover.
6) put two bolts in through the top to help you guide the cover on without smashing the sealant in the wrong place.
hand tighten all bolts then go torque pattern just snugging them up
7) wait to dry (30 mins-1hr depending on your patience or need to drive)
8) After drying snug bolts up just a bit more
9) Add 2-3 quarts (they told me i needed 4.2quarts but it was closer to 3) of 80/90 gear oil. I used Castrol. Just fill till it starts coming out of the drain hole on a level surface.
10) try it out. My diff loved me after 200,000 miles and not one single change.

NOTE TO SELF: THE OLD FLUID STINKS BAD DO NOT GET ANY ON YOU, or if you do use gasoline or brake cleaner to get it off before you touch anything else. Then wipe the can of stuff you used to clean yourself with Just my 2 cents, please tell me if im wrong anyone
Heres my post BTW

https://dodgeforum.com/forum/2nd-gen...500-sport.html

 

BTW they are right check for Limited Slip Differential as you will need synthetic gear oil.

 

don't need syn for the LSD, but you do need a tube of friction modifier for the clutch packs.

 

Aim4squirrels is 100% right,friction modifier is a must if you have a limited slip, you will hear and feel a chatter coming from your rearend without it, I thought I blew my rear diff, it wasnt on an oil change it was just way, way over due, I squeezed a small tube in there and it cleared right up until I could do a gear oil change

 

thanks for clearing that up guys

 

Make sure if you use rtv instead of the gasket that you go to the inside of the bolt holes. I usually do a circle around each bolt for an even seal.

I also put the cover on torqued down and let it sit unfilled for about 2 hours before filing with oil, or replacing the plug.

 

I used royal purple on mine and with the blend of gear lube did not need the additive. I took a temp reading with an IR gun and after the change to Royal Purple the temp dropped 11 degrees and I got a couple more MPG's. Honestly after researching it, I would have saved some bucks if I would have went Mobil 1 - not a whole lot of difference when you go to their websites and compare the chemical make up of both lubes.

 

Acctualy you don't even have to wait for the rtv to dry at all. Gear oil doesn't have detergents, so it doesn't affect it. I usually prefer the high temp orange or ultra black permatex for the diff covers.

seriously though, don't switch to synthetic on a high mileage rear if it doesn't already have it in. I just got another one in this week, 100k on the clock, switched to synthetic about 5k ago, and the pinion bearings are wiped. I wouldn't say it if I didn't commonly see it.