Need help identifying problem. 1998 z71, ext cab, k1500, 5.7L, auto trans., 90K miles. A shimmy/vibration problem causing a washboard effect has developed while driving it in 2WD mode. This ONLY occurs when the drive train is under stress/torque (for example: cruise set and truck is climbing a long incline). Symptoms do not occur under other conditions, such as quick acceleration, passing gear, flat road, etc. Tires are realatively new (15K miles), have been rebalanced and rotated. I have replaced both u joints in the rear drive shaft but problem still occurs. Motor mounts and transmission brackets appear to be intact. Motor performance appears to be fine and does not miss while being reved up to high rpm's in neutral.
ANy tips would be appreciated................Popsgunter