2001 GMC Sierra Differential leaking out the seals

Which seal is leaking?

This is a big job. I can give you the general idea, but recommend that you get a good repair manual that has step by step instructions. There is too much room for error via the net.

First, secure the rear of the vehicle off the ground. Remove the rear tires, drum, rear differential cover, and drain the gear lube. To remove the axles, there are retainers on each side of the differential that will need removed. Once the axles are out, you will have access to the seal. Remove the seal and replace. Make sure to place a thin film of gear lube on the seal before replacing the axle. Reverse the sequence to replace.