Not a stupid question, just got lost in the shuffle.
It is not real easy to replace the "Drive Pinion Seal" on the differential.
You have to take off the driveshaft and expose the large nut holding on the pinion flange that the shaft bolts up to. Before removing the nut, you have to put a torque wrench on it and spin it around a few times so that you can record the torque setting. Then remove the nut. You will have to rig some kind of holder for the flange while you try and loosen the nut. Index mark the flange and end of the threaded punion stem for correct alignment during install. Use a standard 2 jaw puller to help pull off the flange.
Now, using a screwdriver, force the rear axle drive pinion seal metal flange up and strike it with a hammer, exposing the seal. Grap it with a pair of vise-locks and yank it out. Apparantly it needs a very good tug to come out, so you might have to rap the vise-grips with a hammer.
Make sure you lube the new seal with a good bearing grease before installation. You are going to need some kind of "press in" tool for the seal, usually a good piece of PVC pipe of the proper diameter and a rubber mallet work fine. You might also need a pulley installer to get the flange back on (not sure if you should use the nut to tighten it on, but maybe the old nut might work to get it on there. Ford says to always use a new nut on the flange.
How to torque the new nut: Rotate the pinion occasionally to make sure the differential pinion bearings seat correctly. Take frequent differential pinion bearing preload readings by rotating the pinion with a Nm (inch-pound) torque wrench.
If the preload recorded prior to disassembly is lower than the specification for used bearings, then tighten the nut to specification. If the preload recorded prior to disassembly is higher than the specification for used bearings, then tighten the nut to the original reading as recorded.
Torque Settings:
Differential pinion shaft lock bolt (30 Nm / 22 Lb-ft)
Pinion bearing preload (used pinion bearings) 0.9-1.5 Nm / 8-14 Ln-in)
Pinion bearing preload (new pinion bearings) 1.8-3.3 Nm / 16-29 Lb-in)
Regarding the back cover, use RTV, a gasket isn't gonna cut it.