So I think this has been covered but it doesn't come up easily in the search, let alone on google. I've also heard misconceptions and fears over how hard this is to do. Figured I would pot this up now before I forget since I'm already replacing the axle for a full float less then a year late :P
So in a really break way I'm going to show you how to replacing the SET20 bearings (which is a unit bearing with bear, race, seal and retainer, etc...)
What you need: Set20 Bearings, a drill and 1/4 drill bit, chisel + hammer, HEAVY press (mine is 12 tons)
First remove the axle shafts from the truck. Since the nine inch doesn't use C clips, simply using a beep socket to remove the four bolts and a smart tug should free it up.
Next, remove the old retainer. This part is fun and may make you jump. The trick here is to remove the old retainer with out damaging the shaft. I highly recommend agist using a cut off wheel. Agin, no cut off wheels here. The best way is a sharp frill bit, drill two or three holes into the retaining ring about 3/4 deep. No need to go all the way and damage the shaft. If the ring didn't come loose already, grab a flat chisel and strike it squarely across your drilled holes. With a pop it should now be loose.
Next it's the bearing's turn, again, ditch the cut off wheel. Best way to remove it is the press the bearing off. Now, if your cool and have a bearing puller, use that. Otherwise, with my press, the face of the shaft was too wide so I had to get creative. The bearing's inner race might have seized so some encouraging taps with the hammer my help to break it loose. Otherwise it should come off easily.
You can safely press from the face of the axle retainer.
Assembly is in the reverse order. Counter to what's documentation recommends, press the bearing on first then the retainer. It will make your life easier and guaranty a tight fit.
Before reassembly, use new high temp axle grease to fill the little C shaped face of the axle seal (the part that touches the axle). This should help avoid leaks caused by dry contact to the axle at initial setup and after rest. It' also safe to leave the grease from the factory on the bearings.
The order for reassembly should be: outside retainer, Axle seal,bearings, inner retain. The outer race doesn't not need to be installed for this step, but doesn't hurt.
NOTICE: I've opted for removable outer bearing retainer plate, hence the absence in the picture.
Assemble everything, saving the inner retaining ring for last. The bearings can be press off again if you forgot the retainer or the like. Once you install the inner retainer, you should never remove it unless replacing.
The inner retainer is rather hard to install. I used a 2ft cheater to use all 150#s of fat to slide it into place. I little non permanent lube (soap works) with easy installation.
Reinstallation I'll breeze through and say, use the correct puller for the race and apply a light amount of RTV to the outside of the axle seal before assembly.
That's it for now, time for sleep. Cheers!