My 2002 Montero studders like I am driving on a rumble strip when I reach a steady speed of approx 40-45 MPH.

Possibly transmission torque converter lock-up malfunction - OR - an engine misfire at a specific engine load which is applied at 40-45 mph ... Misfire is actually more prevalent that torque converter shudder. Should have qualified personnel perform the required *hands-on* testing so the actual failure can be discovered... Shouldn’t be a problem finding such personnel in “Mechanicsville”... Good luck!