Had both axles replaced about 1-2 months ago and just noticed that the differential is leaking. What did the mechanic do wrong?

 

Drug the splines through the seal (new or used).
Didn't lube the seal with grease (new or used).
Knocked the tensioning spring off (new or used).
New axle has grove, a bur, a dent in sealing surface. Unlikely if a reman, but possible if mishandled.
Didn't replace the seals while there.
Didn't seat the axle fully.

Those come to mind.

 

Well first thing is he did something wrong, so I would take it back ASAP just based on that

 

They claimed that the seals were the problem as they didn't replace them.