Vibration 40 mph in a 280 SE 108 Body 6 cylinder
 

Can feel a vibration about 40 mph, wheels balanced, driveshaft has been rebuilt (thought that was it, u joints were notchy and carrier bearing was shot) any ideas? Not there before 40, can feel it throughout the car.

I know you did it, but 90% of vibrations are wheels and/or tires. Check again? Inside tread separation?

Replace that bad carrier bearing and see what happens.

First and foremost: How's your flex disc?

Bad engine and/or trans mount? Bad bearing in the axle or hubs? Put it on a lift, check all 4 wheels for play & operational smoothness. Check to see if the engine mounts are shot or not. I assume you replaced the bad carrier bearing, but your post isn't clear - just says it was bad. Do that if you haven't!

driveshaft was sent out has new u joints, carrier bearing, and flex disc.

Oh, I see, the carrier bearing was shot. There's an off chance that the driveshaft shop put it together out of phase and/or got the balance wrong. Another possibility is that the tires are balanced, but are out of round, or have a road force imbalance.

same vibration that was there BEFORE I sent the driveshaft out to be rebuilt

If the vibration is more apparent while under acceleration, then it´s probably somewhere in the drive train. It it´s a tire problem, the vibration will be the same with or without acceleration.

Check and/or just replace subframe mounts and motor mounts, and transmission mount and rear axle mount (inside trunk).

These are all critical I have found. All or none.

and everything the others have suggested, too