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The rear driveshaft leaves the transmission or transfer case (if it's four wheel drive) and goes to the rear differential. The driveshaft mounts to the pinion yoke, the pinion yoke is splined to the pinion gear in the differential. This is how the driveshaft transmits power into the differential and eventually to the wheels.
The pinion seal is driven into the differential housing and seals around the pinion gear which spins along with the driveshaft. If the seal fails then it will allow lubricant from the differential to leak out.
The seal will need to be replaced if it's leaking. Depending on how bad it's leaking it may be something that needs immediate attention or it may be ok for just a little while if the fluid level is regularly checked.
I wouldn't recommend going on a 500 mile trip with a leaking pinion seal. If the leak is severe enough or becomes severe enough while driving then the differential could run dry and do thousands of dollars worth of damage. It's much safer to spend the $100-150 to have the seal replaced.