Before you start safely jack the back up and place on jack stands and block the front tires, then take the driveline off.
If it is coming "housing" where the 3rd member bolts up:
1. Take the rear wheels off
2. Remove the rear axles (the 4 bolt flange "under" the brake parts
3. Slide the axle out 4 or 5 inches
4. Unbolt the third member
5. Slide the 3rd member out (if it is stuck take the jack and put some pressure on the yoke until it "pops") then you can put a screw driver and evenly slide it off. Oil is going to drain so be ready to catch it.
6. Clean both surfaces, make sure you don't get debris into the housing or on the gear set.
7. Use some good RTV (I use ultra black) VERY sparingly. You really only just need a very thin coat, don't load it up.
8. Slide the gasket on the studs of the housing (assuming they are studs)
9. Carefully slide the 3rd member on, make sure it goes on square and contacts the surface all at the same time, not chocked.
10. Tighten to the required torque using a cross tightening pattern.
11. Button up the rest of the stuff and you are off.
If is the pinion seal I will have to get back I have to run :-[
