2000 Explorer constant vibration

Hi,
I have a constant vibration that does not change with the different gears in the transmission. I did a stall test (power brake) and the engine does not miss and the torque converter is pulling fine with no vibration present. Start driving down the road and about 15 mph it starts. If you floor it the vibration is there but not nearly as bad, drive a steady speed and it is there but light. However on a light accel it is really bad. I have been under it and shook and twisted every shaft, everything seems to be tight. The tires have been rotated with no change. It really feels like it is in the drive train but I can't figure it out. I would like to know if it is possibly stuck in 4X4. The transfer does not make any engagement type of noise when the switch is changed from auto to 4H (my friends ranger does). With the auto is there anyway to test it, like in gravel by spinning the tires? The auto should kick in normaly so I can't tell. Is there some way to manually disengage or disconnect the 4x4 so I can test drive it?

Thanks for the help.

How about it guys, can anybody tell me how to disconnect it?