I have a 2005 Ford p/u F-250 Superduty. I parked on a decline and I noticed the next day there was a very slow drip from where the drive shaft enters the rear end. Is this just a seal that needs to be replaced or something else?
Yes, this would just be the differential seal leaking. Unfortunately, it is a little harder to replace than just pulling the drive shaft and input yoke and replacing the seal. There is a value called preload which must be applied to the nut which holds the yoke onto the pinion gear. If this is not applied correctly, the rear-end has a propensity to start making a lot of noise. It causes the ring/pinion gears to contact each other correctly which ultimately causes wear on the gears and can cause gear failure if left uncorrected for too long.
This is something you normally want to leave for a shop to do so it gets done correctly in the first place.