Hy lightningtruck, rear axle parts/seals can be very tricky to find. The period you are asking about, many rear axles used a felt rather than a seal and the part numbers in the GM parts book reference the felts. After looking through most of my seal catalogues I think the seal may be National 411330N, the dimensions of the seal are O.D. 3.486", I.D. or shaft size 2.598" width 0.5", hope that helps.
