The other day I had the truck out for the first time since June. All was well. I had to facilitate a rural ropes course and my Golf wasn't quite up to the task of getting me up the muddy, rutted road after all the rain we've had. (I did get a great 3/16" Aluminum skidplate for the car, an 1.5" lift is in the plans)

Drove the truck for a few days. Parked it for a week and noticed a puddle under the rear diff. It's coming below the driveshaft u-joint. I figure the breather was clogged and the pressure blew the seal. Is that a good guess?

 

could be a pinion seal? my old truck leaked from the same spot and it was a pinion seal.

 

Take some brake cleaner and clean off all the spots that it would be leaking from,
Once its clean take it for a drive and let it sit again.
Take a look and see where the leak is located.

 

I think you should change the rear pinion seal IMHO leaks tend to get worse and never get better.
To change it you just have to drop the rear drive shaft and take a 30 MM socket to the pinion nut and remove the pinion flange then use a flat tip screw driver to pop the old seal out. The new one you should apply a bead of sealant around the outside of the new seal gease the inner part of the seal to prevent dry run up it and tap it in gently into the housing make sure it is flush and straight then reassemble it. Try to torque the rear pinion nut to around 200lb lbs check your service manual to make sure on what to torque your to then stake the pinion nut..