Going to replace the seal sooner than later. Have a leak in the rear differential, forward side near the drive shaft. Watched some videos on You Tube and doesn't look to difficult. I am going to replace the gear oil while I'm in there. Any tips??? Thanks in advance, Chris

 

First, I assume the axle is an 8.8 inch version. A tag attached to one of the diff cover bolts tells the story.
Jack up the rear end and pull the wheels and disks/drums to get the axles free. Drop the driveshaft then check the running torque at the pinion shaft nut with a 0-50lb/in dial torque wrench. Note the reading. Take several readings if the running torque is greater than 8lb/in. If less than 8lb/in disregard the readings.
Remove the pinion shaft nut and flange and ****** out the old seal.
A new crushable spacer and pinion shaft nut should be installed, but you can probably do with the old ones.
Drive in the new seal and bolt everything up then check the pinion bearing preload with the 0-50lb/in dial torque wrench.
Using suitable breaker bars tighten the pinion shaft nut a little at a time until the running torque reading equals the measured value. If the original value was less than 8lb/in tighten the nut until the torque is between 8-14lb/in.
If the pinion bearing preload is too light, the axle will be noisy, gear whine. If too great the pinion bearings will go b'ye b'ye. Good luck.
As for lube? Go with 75W140 GL5 full synthetic. (I use Wal-Mart Supertech, about $10.50/qt.) If your axle is a locker, add some friction modifier obtained from Ford or your local parts store.
BTW: The diff cover is sealed to the housing with black silicon rubber. You can use it alone, but I prefer installing a new standard gasket with the black silicon.....that guarantees no leaks.