First step is to get ahold of the service manual - this isn't something you
want to just "wing it" - too many torque specs. and critical measurements.
You'll need a set of torque wrenches (both foot-pound and inch-pound, and
the inch-pound must be either a beam or dial type), a dial indicator and
base, a bearing splitter to pull the old bearing off the pinion, a big pipe
wrench and a length of pipe to hold the yoke while torquing the pinion nut
(replacement for the factory pinion holding tool), and a bearing adjuster
tool to loosen, tighten, and torque the side adjusters (either the real tool
from a diff. shop, an old torsion bar, or some pipe and fittings from the
plumbing section of your hardware store).

If you have all that, or can get it, and are willing to give it a shot,
here's some things that will get you around some of the more difficult parts
in the service manual:

First, use the combination of the pipe wrench around the yoke, with a pipe
slid over the wrench and wedged against the ground or the frame, to hold the
yoke while you remove the pinion nut (it's on there tight).

Measure both the old and new pinion bearings.  They should be the same, but
in case they aren't, you can see how much you need to either add or subtract
from the shim pack underneath the pinion bearing to compensate (for example,
if the new bearing is .002" thinner than the old bearing, you'd add another
.002" of shims before pressing the new bearing onto the pinion.  If the new
bearing is thicker, you'd remove that amount of shim).

The rest of the re-assembly and set-up is pretty much by the book.  When
torquing the pinion nut to set the pre-load, once you get passed the initial
torque to start collapsing the spacer, check the rotational torque with the
inch-pound torque wrench very frequently.  You can easily blow right passed
the recommended rotational torque if you're not careful - then you need to
remove the pinion, put a new spacer on, and start over again.

You're obviously going to replace both pinion bearings and races - if it
were me, I'd put in fresh carrier bearings as well (if you buy a master
install kit for your differential, you'll get all the bearings).  I'd also
get new axle bearings (need a slide hammer with a two-or-three jaw inside
puller attachment to remove them - measure their depth in the axle tube
first, so you know how far in to set the new ones) and axle seals....  cheap
enough, and you're likely to trash the axle seals with the adjuster tool,
anyway.