Vibration with truck in neutral at 45-50 mph

When driving, once I reach 45 miles an hour my truck will start to vibrate badly. Like the entire cabin of the truck is being shot into space and is about to fall apart, but as soon as I reach 50 miles an hour, or slightly thereafter, it will stop and I will have a smooth ride until I have to decelerate back through that speed range. Any ideas as to what might be causing this phenomenon?

This also happens with truck in gear too but to a slightly lesser degree.

 

Wheel balancing is the easiest place to start. Does it feel like it projecting through the steering wheel? Is so, could be front wheels.

 

Automatic or manual? Automatics can have this when it locks up into overdrive gear around 45mph. Not saying that's your answer but something to consider. Maybe time for a fluid flush/change and new trans filter.

Or it could be tire balancing, something in the driveline, etc etc. Vibrations are tricky sometimes. Good luck!

 

Consider driveshaft pinion angle if any lift or sagging rear

 

It is a manual, in neutral and under power it will vibrate but only 45-50 mph