Hi. Thanks for using JA! I will try and help you out.
Most often when a vibration begins on a truck, I first suspect the vibration dampner on the front of the engine has failed. Especially if it has a 2 piece dampner. The tough part is the only way to know if to replace the dampner itself. If you are lick y enough to have another truck that you can swap the dampners on, that is a great idea. If not, the only option is to replace it with a new one.
I the dampner is not the problem, I would next suspect the clutch. I see this from time to time. If yo are able to fell the vibration by reving the engine up to high idle, you can have the transmission removed and the clutch assembly removed, and then run the engine. If the clutch is the problem, you will see the vibration go away.
Please reply back if you need more help.