to replace a rear main seal u have to drop the tranny and take the clutch and everything out. what could happen if its not fixed is that oil could get on ur clutch and then u have bigger problems, but if ur good with ur hands and cars and have the right tools its relatively simple stuff, at least the motor mount should be. if its a manual the rear main seal would be easier and if uve done a couple trannys in ur day it may take u a couple hours for both. u may think about just replacing the clutch and pressure plate, throw out bearing, resurfacing the flywheel, and replacing the pilot bearing if it has one. is it AWD or just front wheel drive?