Try running it with all four tires off the ground. If it still vibrates, it still has to be drive shaft balance. If it's smooth, put four new tires on it.
Jeeps and Chevy Astro 4wd vehicles use full-time transfer cases that run both drive shafts at the same speed all the time. There is a distinction between 4wd and AWD. All-wheel-drive vehicles like the Caravan and Aerostar use a viscous coupling in the middle of one of the drive shafts that allows them to turn at slightly different speeds as in cornering or using slightly different size tires. With the full-time transfer cases, it is imperative that you use four identical size tires. That means same brand, same size, same model, and purchased at the same time. You are not supposed to even buy two new tires this week and another two a week later because there's a chance they were assembled by a different person, from a different mold, or even a different compound of rubber.
This information was in an article in a tire dealer magazine. It was about a lawsuit against the tire retailer for only selling two tires to an Astro 4wd owner. He tore up the transfer case within 800 miles from the forces put on it by the two drive shafts turning at different speeds.
Sunday, July 3rd, 2011 AT 6:52 AM