Check for slightly bent axle? I'm assuming correct p/n? Ensure you didn't nick it with the axle splines on the way in?Replaced the left rear axle seal (again) This is the 2nd one. Axle looks nice and polished no groves, seal was flush with the housing, not overfilled and clear vent. I have no idea why this is happening!.
Seal for 82-88 monte carlo is GM p/n 554631 or 3998519 or Delco 291-309, (or aftermarket equivalent like Timken 8660S) as they are interchangeable.