2001 Mitsubishi Montero Vibration at low speed & rev

My 2001 model Mitsubishi Montero XLS vibrates abd wobbles at 40-50mph when rev counter reads below 2000rpm. When I manual shift the selector to 3 it stops and the rev counter moves up to above 2000rpm. This happens mostly when I am trying to pick up speed slowly at the stated speeds at that rpm