I have a similar issue between 30 and 35 mph. This was caused by my adding an AAL to my rear suspension. This changed raised the rear end about 1 1/2". It seems that the U joints on the 2nd Gen Xterra are very sensative to the angle and this change resulted in the vibration. Don't know if this is what has happened in your case or not, but Nissan carries a kit for this, though it did not completely eliminate the vibration on my X, it did help.