Most common Causes:
- Misfire (usually b/c of bad spark plug[s])
- Dirty Throttle Body
- Small Vacuum Leak
- Bad Fuel
- Bad/Worn Engine Mount
- Bad/Work Transmission Mount
And then some trucks just have a little vibration to them... sometimes they just do it.
There's a buttload of rotating mass inside of the 4.7L and 5.7L Engines... so it shakes.
If you don't have a Check Engine Light then I'd just say keep a close eye on it, and if it gets worse then start with the simple stuff which is Spark Plugs and TB cleaning.