Before you do anything, try to determine WHY the seal is leaking. Sometimes they just fail due to old age, but more likely, the bearing behind the seal has gotten loose. This allows the seal surface to flop around, and the lip of the seal can't keep up. If you just change the seal, and not the bearing, the fix will only be temporary. Before loosening the pinion nut, check for up-and-down, or front-to-back movement of the pinion flange. Either one indicates enough wear to require replacement of the pinion bearings, probably both of them. No slack, OK, now you need to measure bearing preload. Remove at least one axle, or the cross pin and spider gears, to prevent excessive drag from messing up the measurement. Adapt an inch-pound torque wrench to the pinion nut, and see how much torque is required to keep the pinion flange turning. It's generally somewhere in the 10-to-20 in/lb range. Write this value down, you'll need it later.
Remove the pinion nut, with a 3/4" drive socket and a long breaker bar, and probably a big pipe wrench to hold the pinion flange. You'll probably need a long handle, and lots of elbow grease, these things are TIGHT! Remove the pinion flange, being careful not to damage the bearings by hammering on the flange. I like a 2-jaw puller for this operation. Remove the seal, and inspect it for unusual wear or damage caused by the dust shield or foreign matter wrapped around the shaft. I've seen weeds, hay baler string, etc., get between the flange and the seal, and cut one to pieces in a short time.
Reassemble the flange with a new seal, being sure to lube the seal lip liberally with rear end grease. Never install any seal dry. Retighten the pinion nut until the preload drag is the same as it was before disassembly. If it had no preload, try for about 12-15 inch pounds, tightening the pinion nut a little at a time until it measures correctly. Be careful! A small change goers a long way, when you're close to the correct preload! Install the driveshaft, check the lube level, and drive on!
Jerry
