You and your fellow drivers have the right to refuse to operate unsafe equipment.
If you band together and keep writing them up as unsafe, eventually the company will be forced to do something about the problem.
Their usual solution is to just reassign the vehicle to someone who wont complain. If everyone in your center is on board and willing to refuse to drive it, something will get done.
Being out on route and calling in saying "the shimmy just got so bad I cant drive it any more" will force the company to call a tow truck. A few grand in towing bills will get the message across in language they are guaranteed to understand.