Full droop as in the entire axle? Is this for a crawling application? If so, you won't need the full axle to be able to droop, just one side at a time. Even in other applications, really the only time you would see the axle fully drop would be when jumping. Put a limiting strap in the center. This still lets axle fully articulate, while not allowing the diff to drop to far and cause binding issues. It sounds like there are other issues with an extremely short driveshaft, but a limiting strap might work and still let you run the current setup.