Mild Vibration and humming sound after driving 45 minutes on highway.

rear end seal blew out. Got a used rear end with less than 100k on it. My mechanic installed it. Said everything seemed good.

Now thou after you drive the vehicle on highway and or 55 mph roads, after almost an hour the humming noise comes into play. Just like when the original rear end was going out. Sounds and feels the same.

could the replacement rear end only act up once its good and hot?
or could their be an issue with the differential and or transfer case going out only after its good and hot and the fluid is / has thinned out?

driving it locally less then 55mph its fine. No issues.

help please!