Can someone please confer that rear differential oil changes on an 84 300DT is done every 90K. I can't find this maintenance schedule manual. Thanks guys!

I haven't a clue what MB designated for your vehicle. 90K is a bit lofty to my way of thinking. If I were going to let it go that long, I'd probably never change it - maybe just keep it topped up.

Diff. fluid is very easy to change. I do mine once a year at the same time that I go my auto-tranny fluid.