Rough idle in drive but only when sitting at a stoplight

[vt_maverick]

...push way down on the brake pedal and see if the vibration gets worse/better/stops/kills the truck.

That's not exactly comforting Bob.

[Bob Barker]

That's not exactly comforting Bob.

It's not the worse if that happens. It will tell you what the issue is! If the truck kills then it's safe to assume that you used all the available vacuum in the system to press the brake pedal to max, and since it was leaking, you used more than the truck was producing.