1999 Dodge Van vibraton

Good observation. The torque converter spins at the same speed as the engine, so you don't have to kill the engine, just put it in neutral and let it idle. Also, the vibration will come back if you increase rpm while standing still. I had a broken hub on my torque converter that caused a vibration that would come and go every few seconds. That lasted for over three years before it finally broke enough to allow fluid to leak out. Before you dig that far into it, I would suggest you look at the harmonic balancer to see if the hub is loose or cracked, or if the outer ring has shifted on its rubber insulator. A loose outer ring is real common on some Ford truck engines, but it could happen on any engine.

Caradiodoc