I realize this thread has been around for quite a while, and I will check elsewhere/start a new one, but I thought i would jump in and ask this question again, since it doesn't look like it was addressed the first time. Can a slight amount of play in the main shaft on the NP231 cause any driveline vibration under load. I have a 90 YJ, and when the Jeep warms up, it has an annoying vibration from 30 MPH through about 50MPH, regardless of what gear it's in. I have new wheel bearings, ball joints, control arm bushings, sway bar bushings, everything's nice and tight. The problem goes away when the clutch is depressed. The problem goes away when driving in 4WD. Driveshaft checks out perfectly, with new U Joints. Tires are balanced properly and in great shape. Engine and Transmission are new. The ONLY thing I have found is this slight elliptical rotation of the Main slip yoke shaft inside the transfer case when it's in gear. I put the Jeep in gear running with the thing on jack stands, with the rear driveshaft removed. The seal doesn't leak on the tail cone, but this is the ONLY anomaly I can find. I've been suffering this for over 2 years and NO ONE has been able to tell me what it is. Needless to say, it's making me crazy. ANY ideas would be very welcome!