I have a leak in my 83/4 489 Diff. pinion seal. Is this a doable job? I would like to do this in the car

Piece of cake,
You have a beam type torque wrench?
if so pull the driveshaft with said torque wrench measure the torque it takes to spin the yoke. This tells you preload on the bearings

Then pull it apart to change the seal.
re-assemble and, tightening the nut to specs taking into account the before hand readings and try to get back to turning torque / bearing pre-load and you will be fine

I would add mark the position of the nut on the pinion stem so you can reference it when you tighten it up after replacing the seal. We will now hear a few members that will say you cant do it that way and you have to replace the crush sleeve . That will only happen if you over tighten the pinion nut past the previous position(that is why I mark it) just a c-hair past is OK .
In the past 40 years as a tech I have done this easily over 100 times just pay attention when you get close the the marks

Gus

also, apply some gear oil resistant RTV to the pinion yoke splines [after cleaning thoroughly] during assembly to prevent any wicking that may appear to be a seal leak.
don't forget to pack the back side of the seal [where the garter spring is] with wheel bearing grease to prevent popping the spring off when installing the seal.

Gear oil resistant RTV ?

And some very wise aDvICe ! .....,. packing all the back cracks !