HickBoy - super clean under there!!
ALL: How often should this be done as preventive? Mine has the tag and I know it is Safe-T-Track from PHS (and observation ;-D ) On my car this would have been done in 2004 - but only about 13K miles ago.
In my mind, this should not need preventive maintenance - if not leaking. Not sure if PMD ever recommended anything.
The factory manual says "Change lubricant only when necessary to disassemble." I have never changed out rear axle oil in any of my cars. For the manual transmission fluid says the same thing.
The more important thing would be to once and a while check the fill level on the differential and the manual transmission.
I guess like other things, some of us simply feel we need to flush out the rear differential or manual transmission as a preventative measure regardless of what the factory says. Can't see it hurting anything. I suppose with a posi, like an automatic trans, if you drain and refill with fresh fluid it could flush out all the small clutch particles that may be the only thing keeping or helping the friction clutches from slipping if they are just on the verge of wearing completely out.
I just did a gear oil flush & fill on my brother's 1948 International truck with 41,000 miles. Not a lot of miles per say, but it does have a lot of years. Non-synchro transmission so it has to be double clutched or you scrape gears. What came out did not look too good and I did have small metal shards drain out with the oil - from someone scraping gears. So I would say in this instance, changing out the gear oil was indeed needed. But keep in mind that the older gear oils were not as good as today's formulated gear oils, but flushing does get out any metal that could be splashed around with the oil and get into bearings. The '48 manual says to change rear differential fluid every 10,000 miles, but says nothing for the transmission. I plan on flushing the differential and see what pours out of it as well.
So, it may not be necessary to ever flush and fill a differential or manual trans unless you have to pull it apart for some major repair - according to the factory. My 2015 Hyundai manual says my trans fluid should not need changing in its lifetime unless severe driving conditions exist. Define "severe conditions" & "lifetime." I just did a flush at 90,000 miles because it is cheaper than a rebuilt/new transmission, and lifetime in my book is 300,000 plus and somehow I just don't feel the original trans fluid was meant to go 300,000 miles plus without a change. So maybe it simply boils down to "peace of mind" in knowing the gear oils are fresh while the car is in your ownership and you won't have to do it again in your lifetime.