If there are external leaks at the u-joint three seals/gaskets need to be checked and possibly changed. There are originally two cork seals for the ball connector. One a large ring with taper that seals the ball end to the depression on the back of the trans mission. The crush of that seal is determined by the shims (also seals) that set the tension on the ball housing. The housing should have a moderate amount of resistance to movement up and down. The shims and seal determine that. It is described in the Repair Manual. There is also a smaller cork ring between two metal rings where the housing slides on the torque tube. Most of us now use an O-ring to replace the cork seals (rings). The Filling Station has the seals and shims.

If the leakage is from the transmission to the rear end and not on the ground the bushing and seal in the front end of the torque tube need to be checked and probably replaced.