To change the rear main you'd have to pull the tranny, clutch, and flywheel. Unless it's a substantial leak (like you're going through a quart of oil every 1000 miles), I think I'd forget about it. The odds are good that a new seal would leak as well, this is a common problem with 5.0's and difficult to fix for good. Something to do only if you have to replace the clutch anyway.
Oh OK. It isn't bad but it leaks more than I like. I am getting a Spec Stage I put in when I do my gears and driveshaft so I guess I will have that fixed as well. Thanks.