My rear pinion seal started splattering / leaking around 3000 miles. Dealer replaced it and the new one leaked the same. The dealer replaced it again as well as the yoke at 4000 miles and now it only weeps. Dealer didn't check the breather or drivetrain mounts for looseness because the truck was "too new" so I checked these myself and found them to be snug / OK.
I took my truck to another dealer (recommended by TDR members this time) for a second opinion at 11500 miles and they said it was residue from a previous repair. They cleaned everything up real nice and now at 12500 miles the seepage is back!
Although I haven't experienced any noticeable driveline vibration or noises, my next step is to check the driveline angles out. Maybe I'm getting some minor vibration from off-angles that's taking the seal out slowly. I did notice that the center bearing on the rear driveline is surrounded by rubber that has a lot of play to it. Maybe there's enough play to get the angles out of whack when the drivetrain is loaded and unloaded constantly from the massive Cummins torque?!
Aside from this leaking seal the truck is great.