diff bearing life? Should everything be changed when buying a used axle?

[BlitzK5]

When buying a used diff from a wrecker should you repalce all the bearings and seals in the unit or will they survive for many years to come? I hear of people on here scoring dana 60 's and 14bolts with the right gears but do they just throw them underneith there vehicles and drive them around. Do they replace everyhting with a master bearing and seal kit or is it not even need? How long do the bearings survive for? I got a 14bolt from the wreckers but wasnt concerned about the gear ratio because I fugured I should change everything out anyways because its old and used. That way I know It will be reliable and built like new. Im going to add a detroit so i figured I should even change out the ring and pinion since its old and used and probably not a strong as new ones.

 

[89GMCSuburban]

I would if you have it out of the truck. Cheaper and easier to do it out of the truck than to have it inside. $100 for the kit now, or a lot more later.