MS is a very serious degenerative disease and as far as I know UPS jobs are generally pretty taxing on the body, so it was good you made that original decision. It could be something in their internal policies that's preventing you from being rehired, which means you likely won't ever know the exact reason why you're being prevented from being hired again. You can always start over at another similar company and use your experience at UPS to make you a more attractive candidate.