DP/N1 exhaust (rubs driveshaft)

I switched out my Random Tech high flow(BTW one of the wafers was detached inside the cannister after ~15,000 miles) for a straight 3" DP, supposedly a RMM, but it is used, so who knows. anyway once I got it installed, I took it for a spin. On acceleration all was fine, but on de-cel there was a shuderring sound and heavy vibration. The vibration occured on de-cel, sharp turns, and in reverse. After talking to a mechanic friend of mine, i went back under the car, and found the driveshaft donut was rubbing the canniter on my Apexi N1 cat back. I dropped the catback, and bashed in the top of the cannister with a hammer. Viola, problem solved. Anyone else have this problem, or is this peculiar to the DP I purchased? thanks, Carl