Hy Bryan Buser, for some reason I can't open your link, but here goes. The joint between the transmission and the torque tube must be allowed to move a little bit as the rear axle moves up and down. To that end there are "shims" [thin gaskets] to adjust the tension on the joint. There is a cork seal near the o.d. of the bell, over the years of movement the cork can wear, and start leaking, removal of some "shims" will tighten this joint seal up and hopefully stop the leak. The procedure should be outlined in the maintenance manual that you can access through the link you posted. Yes transmission oil can migrate down into the rear axle, if the torque tube seal/bushing is worn, hope that helps.
