ON my new diesel rig, the output shaft leaks gear oil. It only does it when I am driving, and I've replaced the seal. The seal rides good on the driveshaft slip yoke, but I'm still getting a drip while driving. It's bad enough to speckle the back of the truck, and underneath is damp from gear oil. You can see gear oil appearing to leak onto the shiny slip yoke surface, but I can't tell how it is getting past that seal.

Any ideas?

87 Raider 4D56td v5MT1
31's..Basically Stock