If the drive shaft fell out, I would be concerned that it is not bent. Doesn't take much of a dent in a driveshaft to mess it up bad. If there are no dents and the drive shaft is still balanced, which way the driveshaft is connected to the rear axle shouldn't matter. On the rare occasions it does, simply turning the driveshaft 180 degrees may correct the out of balance. I wouldn't expect much help with turning the shaft 180 degrees, but it might help.

You should be able to jack up the rear end on a level surface and get both wheels off the ground with jack stands under the rear axle. Its important the weight of the van is on the axle, and the axle is on the jack stands, and the tires clear the ground by a couple of inches. Start the van and put it in gear. At an idle, while in gear (a helper would be great but it can be done alone), look to be sure the driveshaft is spinning true and not wobbling at the center or either end. If its wobbling, it needs to be rebalanced, or the U joints replaced at the least, but the drive shaft probably needs to be replaced because I suspect its bent. You can not straighten a bent driveshaft. When you have completed the test, shut the van off in gear and let it coast to a stop, then put it in park after the tires have stopped spinning and you are ready to set the van back on its wheels.

If it spins OK at an idle, you can bring it up slowly to about 45 mph, anything out of balance should show up, you will feel the vibration up through the seat and the van. If the vibration gets real bad at any point, stop the test. Be aware that an out of balance vibration could be the tires and wheels, or the driveshaft. By theory, you should be able to bring the speed all the way up to the driving speed you drive, but I simply don't trust jack stands that much, should the stands fail, and the tires hit the ground, the van will take off at whatever speed the speedometer says its going. Not a pretty picture.

Obviously, a bad vibration needs to be corrected before you drive it, but a very minor vibration may be simple tire balance and can be corrected when you have time. I'd start with the tires before I'd try anything with the driveshaft balance. Also be aware that a vibration you feel with the van on jack stands may or may not show up when driving, and sometimes a vibration while driving won't show up with the wheels off the ground. Tire balance or tire problems can often show up like that. Usually a driveshaft balance shows up consistently. Gene