If it's leaking as badly as you say so as to leave a trail down the driveway I'd look somewhere besides the rear-main seal. They don't usually leak that much, whereas a ruptured oil-filter hose or cracked fitting will leak a lot.
It's a steady drip coming off the bell housing about every three seconds when the truck is running. I don't know if that's considered a lot or not. My next idea is to do what the above suggested and see if I can pressure wash the engine and see if I can get a better idea where it's coming from. It's hard to tell now with so much oily matter and grease on the engine and frame.
