it sounds to me exactly like the tail shaft hanger bearing.
i've had it with all of my 4WD mitsi's quite common if they have done a hard 120,000kms+
easy way to check, get under the car in between the front and rear doors, grab hold of the tail shaft before or after the bearing, which bolts to the chassis of the car, wiggle it up and down, if it moves more than 10-15mm its stuffed.
general symptoms are, vibration under acceleration and deceleration, no virbation at constant speed, vibration can sometimes go away under acceleration high in the rev range. more pronounced in second and third gear.
if it is that, its a reasonably easy fix, replace the bearing...