Your rear end is already toast. Don't waste any time or money with it.
The reason you are getting the noise is the same reason you are getting the leak. The pinion bearings are loose, probably due to wear.
The results that Ted got are quite common and I would say that even the best techs are in a real quandry with your condition. The seal is leaking due to the movement of the pinion (maybe a little age helps out). To resecure the pinion would require a rebuild. The pinion is originally tightened by crushing a spacer located between the two bearings, untill the pinion shaft requires a certain degree of force to rotate (usually around 15in-lbs). With the carrier installed (ring) this measurement can not be made. Besides the manufacturers say to never recrush crush sleeves. The bearings that at one time filled the area till they were tight are now loose. That came due to wear and they need to be replaced and reset up.
On cars with no noise the situation is still a real problem. If no noise and no verticle play, I will install a new pinion seal after first marking the nut to the shaft. After replacing the seal tighten the nut to its exact point as original.
Bosch Master, ASE Master, L1
33 years MB technician