2002 Montero Vibration / Rumbling

I just bought a 2002 Montero Sport LS. 41,000 miles. This was a "certified inspected" car with Carfax reporting no problems. I noticed while in AWD and excelerating between 50 & 70 I feel vibration under the "drivers side" floor. Just the drivers side. Seems to be worst what I am on the gas. When I let up on the gas it appears to go away. I tested this is 4H but it didn't seem to do it. ONLY in AWD.
Any advice would be appreciated. Thanks

I just bought a 2002 Montero Sport LS. 41,000 miles. This was a "certified inspected" car with Carfax reporting no problems. I noticed while in AWD and excelerating between 50 & 70 I feel vibration under the "drivers side" floor. Just the drivers side. Seems to be worst what I am on the gas. When I let up on the gas it appears to go away. I tested this is 4H but it didn't seem to do it. ONLY in AWD.
Any advice would be appreciated. Thanks

May be a transfer case or driveshaft/axle balance issue.