So before you tore the rear end down, could you tell there was moisture in the oil?

It has water in it. They all do after 20-25 years unless someone recently had both axle carriers off. You could just pull the rear cover and change all the oil/filter but without pulling the axle carriers off you will never get it all. How much is in it is the $64,000 question. The one I just did was froze up solid when I bought it last January and would not move. Had to drag it on the trailer.

I do not recall being able to tell much about moisture from just the dipstick. The overall condition of that tractor screamed that the rear oil and filter needed to be changed. Our plan involved changing the seals, gaskets and of course drill the drain holes. The large amount of water we found in the axles did amaze me and confirmed the need for the drain holes.

If the rear is in good condition and you don't need to pull it for other reasons I would suggest that you at least pull the rear cover, pull the axles, seals, wheel bearings and use a suction gun to suck the water out of the bottom of the axle tubes. It's not a big deal to do this. Reason I say this is that I have seen axle tubes that had water in them freeze and crack. Last time I checked Michigan weather gets below freezing on occasion. Couple pics of what came out of the tubes on a 1772 I worked on. Used a vacuum brake bleeder to suck the crud out.

So when you open up the rear, how do you remove the axles? Is there a clip in the rear end holding the axles in? Pull the clip and slide the axles out? Of course, if you do that, I'd consider taking the axle housings off and drilling the drain holes, but if I remember from your pictures, you had some bolts break off when you did that.