Check the front driveshaft, shake it about and see if you have any bad U-Joints?
Also make sure your tires front and back are the same diameter. I had bad noises with my 4x4 when I bought my truck because the tires wore unevenly. New tires cleared that up.
Also, I had a horrid vibration @ 45mp+ mph too. Turned out the inner fender plastic was bending back in the front passenger, hitting the tire and making noise.