This is in regard to my 99 Ranger with 4WD.
After I hit a bump or an expansion joint in the road, it starts sounding like I am driving over one of those grated bridges. A real smooth low vibration. This stops when I slow to under 28mph. It also "sometimes" stops when I hit another bump. I don't feel it in the steering wheel nor brakes, but I get the sense that it is somewhere in the front suspension or driveshaft. Oh...I do not use the 4WD feature. Wondering is there is some typical thing that might happen with this particular truck. I've not had it long enough to be very familiar with it. Thanks.

 

I also have a 99 and I have the same problem! Let me know if you have found a solution! Thanks

 

Problem solved. One of the 4WD hubs had gone bad before I got the vehicle and had been removed and not replaced. This evidently allowed parts in the 4WD mechanism to flop around enough that parts were making contact...parts that would normally be held apart by the hub. (sorry...I don't know the names of the parts on 4WD mechanisms) Anyway...a new front hub bearing assembly was installed, a new retaining ring was installed, and manual locking hubs were installed. Back in service with no vibration and 4WD capabilities are restored.