The D35 pinion seal is just a bit tougher than the M15 to handle offroad use due to the excess dirt that on-road cars don't see. They learned from having to change so many on the Eagles under warranty and Dana did the upgrade when they redid the axle for Jeep use.
If I had a few more dollars to spend on my projects, I would take a Model 20 from a Grand Wagoneer or narrow track Cherokee/Wagoneer and have new shafts made with the correct bolt pattern and offset to use XJ 10" drums or possibly find 11". The stock ratios kind of suck though. 3.31 or 2.72 was most common after '80.