Odd vibrations are sometimes difficult to isolate but there are a few things to look at.
1) Are the front tyres wearing evenly or are there some scoops appearing around the tyre? This could point to bad wheel alignment.
2) Does it look like any wheel balancing weights are now missing from the wheels?
3) Is the sound louder or more noticeable in left hand turns than right (or vice versa) ? If the sound is louder while making a left hand turn, chances are the right hand wheel bearing is worn.
4) Are the tie rod ends firm or is there a small amount of slop in these?
Climb under the vehicle.
5) Are the front CV boots whole or are there splits in them? Move them about. If there's signs of grease on the outside, chances are they've been leaking and it's possible that dirt has gotten in.
6) Are both front and rear drive shafts sitting (very) firmly or can you give them (on either end of each shaft) any sort of wiggle? Worn uni joints are often a cause of vibration.
7) Is there any sign of fluid leaking from the rear diff? The seal at the front can be damaged and start leaking if the bearing there starts to wear.