DRIVE LINE PROBLEM, HELP PLEASE

I HAVE A 2000 F-150, ITS GOT THE 4.6L V8, I HAD A NEW DRIVE LINE MADE FOR IT WHICH IS 3 1/2" ROUND, AND ABOUT 81" LONG.

I HAD IT IN THE TRUCK BEFORE I TOOK OUT MY GAS TANK TO PUT IN THE NEW CUSTOM GAS TANK.

BUT NOW WHEN I DRIVE OVER 65MPH IT VIBRATES REAL BAD, ANYONE HAVE ANY IDEAS ON WHAT THIS COULD BE? OR WHY IT COULD ALL OF A SUDDEN BE VIBRATING?

I DIDNT DENT, DING, DROP, OR THROW IT. SO IM HELLA CONFUSED.

HELP PLEASE