My opinion is, you'll spend far more time fiddling with the parts by trying to do this job in the car. Pull the ZF, clean it up if necessary and be done with it. By spending far more time than I should have, I converted my ZF to 4 short bolts in place of the stock two long bolts- in the car- and found that you have to lift the ZF to exactly the right height for the long bolts to go thru the provided holes in the inner fenders. Then you find that the exhaust pipe on the wheel side of the inner panel are now in the way and also must also be removed.
In the end, you''ll do everything needed to pull the ZF except lift it out and work much more conveniently on a bench. The seals are std parts that are/were available at King Bearing or other supply stores. I no longer have it but the seal number will be on the old seal, or simply take the old seal down to the store and have the counter guys match it up. There are also big shims and a giant snap-ring inside so real snap-ring pliers will be necessary to do this job correctly without bloodshed.
I suggest doing both side seals, as parts like this tend to wear out in sets and most of the labor is getting the ZF out. The front input shaft seal is also a std part #. I found one leaking slightly in a ZF that had been "sealed" into the nose piece using rock-hard JB-Weld or Dev-Con that had to be chipped out with punches, chisels and a small hone. God-save-us from inexperienced home innovators!