Good to hear reply's! Back about 5,200 miles ago I replaced the pinion seal myself. It was pretty straight forwards, I even found a Youtube video. The only issue with it was, like everything else on the underside of my RV, RUST! The worst part was removing the fill plug from the rear inspection plate when I attempted to top off the rear end lubrication. Someone had rounded the square hole out. I ended up removing the entire rear inspection plate and it wasn't until I heated the plug red hot with an oxyacetylene torch that I finally got the damn thing unscrewed. I replaced it with a cast iron plumbing pipe plug with a huge square external head. I put Permatex gasket cement on the threads when I reinstalled the new plug so that it'd never stick again (a saltwater boating trick I'd learned).
I'll check the axle vent to make certain that it's open. With all the rust and such on the underside it could easily be clogged.
Fire up, if in the future the leak gets worse and I decide to replace the seal I'll give you a shout. I have a Chilton's manual and they make servicing the Dana 80 look a bit tricky with shims and specific torque specs and such. The Dana 60 axle looks much easier to service BTW. Do you have a Dana 80?
Steve