You have to pull the tranny or engine out first, and remove the flexplate/flywheel. Then unbolt the seal housing from the block and pan, being very careful not to tear up the pan gasket, or you'll have to pull the pan, too. If the timing belt is due for a change, it could be done at the same time, but you'll have no easier a time of it while doing the rear seal. I'd do the belt and front seal together. Make sure it's not a leaking valve cover first! Overtightened covers leak due to warping, often at the rear. Get the engine degreased at the quarter car wash, let it dry on the way home, and use baby powder to look for leak tracks on the rear of the block.