activematrix

I recently picked up a used 2007 STX rwd with the 4.6l V8. I made the dealer rebuild the differential because it was making a grinding noise. I just got it back, and I put 500 lbs of sand in the back for the impending winter storm, and I now notice a vibration that I didn't feel before I put the weight in. It is at all speeds, but seems like it gets a bit worse the faster I go. It is a very fine vibration, not like a warped rotor or anything. Anyone have any ideas? I still have 70 days or so on my bumper to bumper warrantee, and I want to make sure it is tip top before that runs out.