When I removed the plug to check the differential oil level, I lost possibly a couple gallons before I could get a pan under it. How likely is it that hydraulic oil from the brakes is getting in the differential? The transmission was at "add" last year and I topped it up with about 1/2 gallon. Haven't checked the differential level since before that. I had to "pump" the brakes after it sat for a few weeks recently but the brakes seem fine now. I run Hytran in everything. I think I'll run it awhile and see if the level builds back up.
A bud pointed out that the rear axle vent had to be plugged in order for the oil to blow out under pressure when I removed the level plug. I checked it and sure enough it was totally encased in a mass of oil-soaked mud. I cleaned everything and replaced the vent which was still stopped up even after cleaning. I ran a couple hours yesterday for a small job and the brakes feel noticeably better. I'm not sure of any connection but there was definitively a difference. My bud says dropping the rear axle and rebuilding the brakes is in my future but to check to level frequently and if not building up too quickly I should get by until the winter. The rear axle breather has an internal standpipe which possibly serves to guard against excess oil level.