Hi all, mostly the contamination comes from wear and tear, (or a permanently vented broken breather) if you think of the moving parts you have, diff, sun gears planet gears, bearings, shims etc. try looking for wear in your diff, thats what the oil is mostly there for. There shouldnt be any contamination from your swivel housings unless the seals have gone or excessive lift on the drive shafts, also the swivel housings shouldnt have grease in them ( just oil ). I have found once bearings start to wear and pit there is no going back. Most vehicles have excessive lift and wear its something you might have to live with or spend money.
The swivel housings should have a filler plug, turn the steering to hard lock and use a
probe to check the level.
I hope this helps