I’ve replaced a pinion seal.
I did it with the diff. in situ though which allowed for using the handbrake and foot brake to ‘lock’ the diff. while working. Then marked the pinion nut relative to the diff itself. A long breaker bar was all I needed to slacken the nut. Count the turns off as you remove it, because you want to be able to put it back in exactly the same position later.
Once the nut and washer are removed I found the drive flange simply slide off by hand. This may depend on age and or previous maintenance though. The splines are supposed to be sealed. You may find that it is stuck with sealer. If so you’ll need a puller to shift the flange.
With care you can remove the old pinion seal. I’d advise protecting the pinion shaft splines before starting on the seal. The slightest ding in the splines and the drive flange won’t go back on. A split piece of PVC drain pipe or similar will do.
The old pinion seal took some brute force. It defied all attempts to prise it out. I resorted to a slide hammer and even then it took some hard blows to shift it. The outer edge of the pinion seal was coated with some sort of sealant before being fitted at the factory and you can expect it to be well gummed in.
The pinion seal has a fine metal spring around the inside of the rubber lip. Take a look at a new seal before you begin. Make sure you remove the old one. On reassembly smear some bearing grease into the trough where that spring sits to help keep it in place.
Then smear the outer edge of the new seal with some anaerobic sealant such as Loctite 574. Before tapping it in place.
(After slide hammering out the old seal, adding more setting sealant on the new seal doesn’t feel right, but it has to be done.)
Likewise, clean the splines and add some anaerobic sealant to them too, otherwise oil will track down the length of the splines and leak into the cavity where the nut resides before being flung out of the gap between the input flange and the propshaft mount.
Lube the new seal and the outer of the input flange with some gear oil before you push it in.
When it comes to replacing the nut. Recount the turns back on, then tighten until the marks you made earlier realign. That way the preload should be the same as it was before.
Put some thread lock on the nut before tightening too.
The only difficulty I can see with doing this off the car is that it may take some ingenuity to prevent the diff from turning - as you can’t use the brakes to help here. You might get away with this technique if you have a power impact driver.
This guy is well worth watching too. It will give you a good idea of the parts and how they all go together.