you might be able to use an additive to rejuvinate your seal. These engines are tricky to seal. You will have to drop the oil pan. Does the engine have any other problems? If it does it might be a good time for a complete overhaul. If it is not leaking to bad, you could put one of those oil pans under it when you park, and carry a couple of courts of oil behind the seat. Check your oil level at every fill up.
If these old trucks aren't leaking oil, they are probably out of it.
Replacing the rear seal is quite a job, especially if you don't want it to leak again
