Yeah the D35 rubber plug always leaks and get the axle nasty after a while….pretty normal. I’d just clean it up periodically. I always considered getting a different 35 diff cover but never really found one that kept the stock height fill plug for proper oil level. Eventually the 35 got replaced by a 44 so no longer on my mind.
Probably be a good idea to have All your gear train serviced.
Transmission, Transfer case, U joints and the Differentials.
By the looks of it, it's probably never been done, they often neglected.