thumping at speed over 40 from rear end

Hello, This is likely a condition called 'tramp'. If you are experiencing tramp, the wheel is literally bouncing down the road due to worn out rear shocks. Have someone drive next to you when it occurs, or just feel the treads on the tire. You can see the wheel bouncing, or you can feel that the individual tread squares are sitting at different heights. This is called cupping, and will make your safari sound like you are actually on a safari (rumbles like a dirt road). If you have other questions, or need more information, let me know, otherwise, here is a list of certified shops in your area with the training to repair your vehicle: http://repairpal.com/gmc-within-10-miles-of-southgate-michigan-48195