The engine and transmission does have to come out as a complete unit through the bottom of the car... It is not much more difficult than doing it on a FWD car as long as you have a litft. The extra effort is a few bolts for the rear driveshaft, a few more supports and stuff for the transfer case and a more difficult re-assembly of the motor to the transmission. At max, I would say it is an extra hour of work for an experienced mechanic with the right tools.
www.alltrac.net has a good writeup for engine removal in the general maintenance section of the forums as a guide to what has to happen.
-Charlie