driveshaft question

I have a 2002 f350 supecab dually 2wd. After replacing brakes, I crawled underneath to look at the driveshaft and rear end to check the u-joints. I have a 2 piece shaft. Behind the yoke to the rear of the intermediate bearing for the shaft, there is a rubber sleeve around the shaft. What is this covering? It has stainles ties holding it on.. not hose clamps. There seems to be some play at this joint.