There is a problem with a DIY mechanic of any level of experience doing a seal replacement on the final drives. There was a time we got the same gasket the units were built with and we did this. Now the gasket, #13 in the exploded diagram, is not the same material. It is a different thickness. This gasket will effect the position of the cover which will change the spacing for the large bearing pre-load. The final drive has to be reshimed after replacing the gasket and this is done to get at the seal. Failure to properly shim the bearing, notice #6 in the top drawing, that is a shim, will cause failure of the unit. In this case a final drive unit failure is not something that can be fixed. It means the unit can die.
Fine a dandy you think? But in order to do the shimming special tools are needed. I don't think you can even buy them. I've never seen a picture of what they look like.
The small seal on the pinion side may be doable by a home shop mechanic but you will need some other special tools. Just a couple of sockets I think.
Replace the O-ring seals in the brake cam hole and this will probably fix it. If not prepare to send it to somebody who can fix it.
You can probably return the parts for credit. There are plenty of other things to work on.