It really is a simple swap.  Looking up at the rear of the center skid at the transfer case, you can see where the speed sensor is mounted.  There is only one bolt that secures the clip holding the sensor down, and after removal you can pull the assembly straight out.  Expect a little fluid to come down with it, so before you reinstall the gear, take the opportunity to make sure your level is back up.

I've found the easiest way to get fluid in hard to reach holes that funnels may not work for is to keep a gear oil top around.  It will thread right onto most fluid top threads and then the bottle can be tilted into position and the fluid directly squirted in.

Now you can separate the gear by pulling it directly away from and out of the sensor assembly.  Check your o-ring to make sure it is still intact (Dana's was brittle and needed to be replaced- another dealer item).

The new gear can now take it's place, but there is a small trick to replacing the assembly.  First you want to make sure you feel the gear slide in correctly, meshing with its counterpart in the t-case.  Second, before securing, take notice of the numbers etched on side of the lower half of the sensor.  You should see numbers that corrolate with different gear sizes.  You want to make sure to "clock" it to match the teeth number of the gear you chose.  To do this you will need to rotate the sensor until your magic number aligns itself with the stamped dot at 6-o'clock on the t-case hole rim; simple enough to miss, but you're speedo won't get its read-out without it.

If reinstalled correctly, everything should line up and your speedometer should read your speed, but before putting all your tools away, take a little drive down the block to make sure everything is meshing.