It may also be leaking between the seal and the housing in case previous seals were not carefully replaced. Don't know about the yoke, but these seals do fail often and I would not be surprised if stock yoke manufacturing tolerances are large.
there is an revised seal that came out in 02,that fixes this most of the time, you have to ask for the updated one or you get the old style. I know I had the same prob. I have a friend that is an parts manager at an GM dealer he told me about it.
Repair it yourself. Most likely its not the seal itself but seepage from in between the yoke and the pinion. GM doesnt like using RTV on the threaded end of the pinion gear and rear face of the yoke washer. Try sealing those two things and your problem should be gone.