I have a 98 Blazer 4x4 and have recently had the rear seals and bearings replaced on the axle because one seal was leaking. The rear axle is leaking again from the seal. How would I tell if the axles or tubes are damaged if there is no apparent damage on the outside? The blazer rides smooth, no matter the speed.

 

Whenever I remove the axles I take a wire brush and steel wool to the rusty part of the axle shaft where it meets the seal. This keeps that rough surface from damaging the seal. Also rub gear lube on the shaft to keep the seal from being damaged when the axle slides in. Hopefully cleaning that up will stop your leak.

 

What he said. Also examine the axles for a groove worn into them where the seals ride. If it is deep it can leak.