Temporarily Unmounting Garage Door Opener for Repair
I found the answer at http://www.thathomesite.com/forums/load/garages/msg0321575526348.html?38 - here it is:

First off you don't have to replace the worm gear. Second, you need to make sure that the bushings in the gear kit are not worn. You can tell by looking and the very top of the unit, you will see ground up metal, or on the top of your drive gear will have a bunch of ground up metal. If this is the case you will need to get a 41C4220A which is the complete gear kit.

Otherwise replacing the gear is a cinch.


Unplug the opener and pull the cover
off.
You will see two roll pins in the
shaft that the gear slides onto.
Position the gear so that you can
use a 5/32nd punch to drive out the
bottom roll pin. Your gear maybe
worn enough that you can grab it by
hand and position it, or you may
have to hit the opener button and
stop it so that it is in position.
At the bottom of that shaft you have
a smaller gear that drives the limit
assembly. Pull the triton shaped
keeper out and slide it off the
shaft.
Remove the limit switches. They snap
into the bottom of the motor
housing. You remove them by lightly
squeezing them together on the end
closest the gear you just removed.
Leave all the wires connected to the
limit switches. Unplug the RPM
Sensor. It is located to the left of
that big black cup on the back of
the motor shaft, it has a four pin
connector.
On the left and right side of the
motor housing you will see four
black bolts holding the motor
housing up. Use a 5/16th socket and
remove all four of them.
Slide the motor housing off the
shaft (you can let the motor hang)
and then the gear slides off.
Lube up the new gear with the
grease in the box and reverse the
above procedure.
MAKE SURE YOU HAVE THE BLACK CUP ON THE BACK OF THE MOTOR SHAFT PRESSED ALL THE WAY ON AND THAT THE FOUR PIN CIRCUIT BOARD NEXT TO IT IS PLUGGED IN, OTHERWISE IT WON'T WORK PROPERLY.*


Check the bushing in the motor housing that the gear shaft slides through, sometimes this has to be replaced but, very rarely in my experiences.

Don't worry about using anything else in that box.