vibration under explorer ford

The Blue Beast, formerly the rust bucket, has a new vibration problem. 91 Explorer 4.0 4WD The problem: At any speed above 25ish I get a nice "VVVVUUURRRTTTT VVVVUUURRRTTT" vibration that I can both hear and feel. It goes in time with the vehicle speed, increasing almost to a steady humm at...

Just a small problem I'd like to fix myself if possible. So when driving sometimes, there will be a vibration and a little bit of a rattle, also under my vehicle. It's usually when I stop at a red light and only last I'd say any where from 3-8 Seconds.. What could the problem possibly be and...