Rear Gear Oil Change and New Seal 1973 Dart

It's a little easier if you just want to change the gear oil and not the pinion seal. If the pinion seal (around the area where the driveshaft attaches to the rear diff) is not leaking, there's no need to go into the hassle of replacing the pinion seal.

If it's an 8 1/4 (8.25) you remove the differential cover off the back and the fluid will drain out. Remove any remaining gasket material on the cover and the differential housing and replace the cover gasket with a new one. It should take about 2 quarts of 85W90 and if you have a locking rear, you'll want to add 6 ounces of limited-slip additive before the gear oil. There's a fill hole about halfway up the side of the differential. Add gear oil until it runs out the hole!

...and in the words of our friend Del (67Dart273), head over to MyMopar - Mopar Forums & Information - Service Manuals and download a service manual.