There is a chattering sound coming from my drive train when idling.
In my 1994 Toyota pickup there is a chattering sound coming from my drive train whenever I'm at idle...
U-joints don't move at idle.
You'll need to be more specific with the question. Get out and wander around the truck, open the hood, crawl underneath. At least narrow down the general location of the noise. It could be exhaust deterioration or a mount, loose mounts around the drivetrain, worn out motor mount, or anything that should be attached to the motor (which vibrates at idle).
My first guess would be that the exhaust is hitting something at idle when vibration is usually greatest, but it could be anything.