You do not have to remove the pinion gear. All replacement work is done from the front side of the axle housing. Disconnect the drive shaft, remove the pinion attachment nut and slide out the old pinion flange assembly. I do not replace the crush sleeve, so if you decide not too make sure all spacers/sleeve stay or go back in place. Install the new pinion flange assembly and nut, torque to correct value and reinstall the drive shaft.
Lee- The flange and seal can be replaced without removing the pinion gear but you may not be able to get to the crush sleeve. The best bet would be to pull the pinion and replace the crush sleeve with a new one but...Since I had recently rebuilt my axle when I replaced the flange, I first tried installing the new flange with the old crush sleeve in place and tightening to a slightly higher torque than I used the first time around. It's impossible to correctly measure bearing preload with the differential installed, but I checked everything before disassembly and again afterwards and I was lucky enough to not have to replace the crush sleeve. On a factory axle or an axle someone else put together, I would definitely pull the pinion and replace the crush sleeve.