Vibration at Deceleration

Make absolutely sure you have the correct pinion angle and that it's equal and opposite at the tranny end and the gear end. If one end is 3 degrees down, the other should be 3 degrees up. I found out the importance of that on my race truck - first time on track after installing a Ford 9 inch, I broke a driveshaft, tore out the rear of the tranny, and wrecked my truck. It doesn't get talked about much but it is very important. Also, have the driveshaft shop recheck the balance. I've had them say it's balanced and upon rechecking find out it's way off.