I installed a diff flange seal before where I got it too cocked eyed while installing it.  When I was done, I ended up fubaring the seal and it leaked almost immediately. (not alot, but it leaked) 

Pulled it back out.  Looked fine enough.  Replaced it, this time using sealant (grey silicone) around outside of metal seal, used grease on the rubber seal, and grey silicone on the splines(I put it on the flange side so extra sealant would be pushed out of the t-case on install).  Made sure to install it without cocking it and have had no worries since.

When you grab the flange how much is it moving?  Up to down and in and out?

Another thing to consider.  Is your driveshaft balanced?  My driveshaft was so wacked that @ about 70mph it would viberate enough to allow gear oil to slip out of the rear t-case flange.