drone- very slight vibration - not exhaust

[Mondo]

Hi,
I have this weird drone that comes up around 30mph & 60mph (especially when cruising at those speeds). At first I thought it was exhaust/rpm/transmission thing but I've slipped the truck into neutral and the drone is still there as it coasts thru the 30/60mph range.
When I put on the brakes there is no difference. There's no shimmy or anything like that. Just a slight vibration. In fact, if you didn't know the truck before you probably wouldn't notice.
I think it has to do with the brakes/wheels somehow as I've recently had a brake job (new rotors/pads) put on. I have the disc conversion and wonder if they somehow used the wrong year parts (I don't know if they're early '70 or later '71-'72 parts). Or if that's even possible.
any ideas?