2006 CM870 vibration in floor 6th gear

[Chamberpains]

With the new tid bit of information, it pulsates in 6th gear, I'm gonna recommend a thorough check of the rearends and drive tires. Either a dyno or get it up off the ground and drive it. Listen for a rearend grinding or look for flatspotted or out of round tires. Including the trailer tires. I just caught a set of my trailer tires that were slightly flat spotted sending a vibration up to the truck around around 30-50 mph. Also I rescently lost a rearend that made vibrations at the same speeds.

EDIT** I just went and re-read everything again. This complaint strongly points to rear engine mount bushings as others suggested. You won't always be able to see them worn out. Sometimes the rubber gets soft or the bolts wear in to the bushing and allows alot of movement with out looking shreaded or overly compressed.
Also you said

2007 Pete with an 06 cm870. Truck has just over 423k on it. Valves adjusted a few months ago, driveshaft/u joints are all solid..

I'm thinking if the truck, not the engine, is that old and only has that many miles on it. Then the engine mount bushing may have dry rotted. Trucks that old with that little of mileage usually sit around a long time not moving. That leads to a lot of dry rotted rubber components.

Read on if you wanna see how I search for any kind of vibration
END EDIT**

The speed of a vibration or pulse will often give away whats causing it. If its fast, then it has to be something that spins fast or can happen often. Drive shaft, tranny, clutch, engine, really messed up tire...

If it's slower, then it points to something that spins slower. Rear gears and bearings and flat spotted, mis mounted or out of round tires.

Why would it only happen at a certain speed? 2 things, either it stops occuring above and below those speeds or that because vibration travels in waves exactly like sound travels. You can't hear really low sounds and you can't hear really high sounds. That same principal applies to feeling a vibration in a vehicle. You can't feel it when its too slow most of the time because it's not happening or it doesn't have enough force to travel up to where you are or it's frequency is not often enough to be recognized. And if its too fast then its either corrected itself (often times rear ends correct themselves)or its vibrating fast enough that it can't be felt cause the frequency is too fast for the components around it to carry the vibration.

If you know what components with a flaw in it can make the vibration in sync. or in time with when you feel it that will also narrow most searches down quickly. Like a flat spotted tire will be once every rotation or once every couple seconds. Or a u-joint will be very rapid and get faster as the truck goes faster.

One last thing I do to narrow down a search while driving at speed is, if you can feel it with your hands in the steering wheel, then its in the steering system. If you can feel it in your butt(or the seat) than its drive line(drive shafts on back to the tires, including the trailer tires) if you feel it in the peddles its usually engine clutch or transmission.