Mark the nut and the shaft so you can count the number of turns on the nut when you remove it.

Get the pinion seal out, it can be difficult, I like a slide hammer with a screwpull, comes right out.

Tap the new one in, an empty can of peas works well, generally destroys the can.

Put grease behind the seal on the coil spring before you put the flange on....sometimes it's easier to do this before you install the seal. Put gear lube on the pinion flange and the seal before you put the flange back on. Use antiseize on the splines shaft before you put the flange back on...someone might want to get it off someday.

I also use blue loctite on the flange nut.

The volvo book calls for tightening the flange until you get a specified inch-lb rotating torque, but that can be a little esoteric for beginners.

Just be careful and don't overtorque that crush sleeve, or you'll be replacing the sleeve and learning the rotating torque measurement technique!

My dealer wanted $180 to do the seal replacement, the seal is about $26 list. You should be able to get the seal for at least 15-20% off and do the job yourself in an hour or two tops.