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

Check the the carrier bearing supporting the 2 piece drive shaft.

Alan, my pickup doesn't have the 2 piece drive shaft but thanks for the tip.

Check your tortion arm mounts on each side. They are roughly under the driver's door and are on the inside of the frame rail, one on each side.

I just had the same problems somewhat rectified on my 99' silverado. Turns out the drive shaft was somewhat out of balance after having my U-Joints replaced.
Just some food for thought for yah.

After rereading your post, a thought occurred to me. When was the last time you did a tune-up? Your engine could be missing under load - causes could be wires and/or cap and rotor.

I had the same kind of thing happen to me on an 88 full size jimmy i used to have and the first thing that i tried was replacing the u-joints which did not solve the problem. It turns out that the geard in the rear end were about to puke out the cover.......the spiders to be more exact. I suggest pulling off the cover and inspect your gears closely.

Sounds like torque converter chatter. Add "shudder fix" which can be bought at any transmission supply house..
Tim
Trans tech 25 years