Breather tube runs along the top of the axle on the passenger side and terminates somewhere near the fender. Don't remember exactly. You can open the fill plug on the diff, and pull the dust cover off of the vent tube and (use a low pressure compressed air) and see if you can feel the air coming out of the fill plug. Or if you're concerned about getting debris in the diff, pull the vent tube off at the diff and blow the air up towards the end. But I think it's unlikely that the tube is blocked.
There can be four sources of leaks -
1 Either seal at the driveaxles. If these were leaking you would notice the oil near the wheels.
2 From the gasket between the cover and the diff. housing. I don't remember if theres room to remove the cover while diff. is still in vehicle. If not, the whole unit isn't hard to remove.
3 From the pinion seals where the yoke bolts on to the differential. On this truck there are two pinion seals, one on the inside and one on the outside. If this is your leak oil will be coming from between diff housing and back of yoke. Complicated to replace.
4 Or not from the pinion seals, but like mine: the fluid is seeping up the splines and actually leaking through on the driveshaft side of the yoke, soaking the universal joint. Will still require disassembly of the diff so might as well replace pinion seals too.
I've been told that these units are VERY robust and not prone to leakage or failure except from abuse/misuse i.e. the previous owner of mine had mismatched tires overheating the whole drivetrain.
I'm not trying to say your abusing your truck I'm just repeating advice that was given to me by a 4wd driveline expert. The expys have a very hardy driveline and failure from normal use is almost unheard of.
Explorers are a different story lol...
