Glad you brought this up, I've been having this exact same issue. Are you running an aftermarket driveshaft? I've got a shaftmasters under there now and the vibration has been there since it was installed. I turned the driveshaft on the flange to see if that would help but it didn't change much. It wasn't really bad enough to cause concern at first so I just let it go, but since then I've replaced the output seal once already and now 2/3000 miles later it's leaking again. I'm starting to wonder if that bearing is worn out as well from vibration in the DS...I'll definitely be following this to see what you come up with and if you decide to change it out. Hopefully it's not too big of a pain...
Edit: Nevermind, I see you've already got a thread started on this.