Probably several things , like motor mounts, cab mounts and such. The fuel
injection system has a lot to do with it. With age the injectors and the
fuel pump wear. The cylinder balance changes and you have irregular fuel
outputs to each cylinder. New engines which are more stringent emission wise
are new and have more exact fuel outputs , thus a smoother engine.
Once had a new Mack exhibit right front tire imbalance. Replaced tire and
rotor. Nothing helped. Around 45 mph the truck would start vibrating. Pulled
injection pump and the fuel balance was off. The engine would rock in it's
mounts giving you the sensation the right tire was bad.