It's a seal, not a gasket, you need to remove the rear drive shafts to get to them
the discription about jacking car up ect sounds like ur Reading about the center diff( witch would explane the gasket talk)
Drain rear diff oil(1/2 inch drive bar) jack up on diff, axel stands on sills jacking points. Mark up inner lateral links an unbolt, leaver shafts out. I use a biggish screw driver to pick the seals out. Gently tap in seals (smear some oil on the seal before you insert the driveshaft) also make sure the shafts go right the way home . Make sure the lateral links go back to ur marks if it will affect you geometry . Oh also before removing the shafts give the serounding area a clean you don't want an dirt or crap getting in.
Refill oil the top 1/2 square untill level(make sure car is level at this point ) if you get stuck give me a call an will talk you through it