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Update: the buzz was in fact coming from the splines in the rear prop shaft. The wobble there was getting pretty bad. Bought a NOS shaft complete and bolted it on. Now the drive train is buttery smooth.

With the clutch in, and when the drivetrain is not under load (between Accel and Decel) I get a pretty big vibration which sounds like a major buzz from underneath, but it goes through the whole Jeep. Get the noise and vibration between gear with the clutch out, too. Seems like it's from the rear end possibly the rear drive shaft? Does the manual cover this type of troubleshooting? I'm guessing it's something to do with the rear driveshaft. Any way to adjust it to make the noise (wear) go away?[/i]

Probably not the driveshaft - such as a bent driveshaft or bad universal joint bearings - you would get vibration all the time the driveshaft was turning, not just when under load. But it's easy to inspect for problems and eliminate the potential that it is contributing to vibration. Yes, its time to look at the manual.

It is best to get it on a lift so you can run it and engage and disengage drivetrain while you inspect and listen underneath. Once you narrow your focus to a specific area or component then final troubleshooting in the book will resolve the issue.

Thanks guys. I'll get it up off the ground and see what I can with it running. It's quite a rattle with the clutch in, and buzzes the entire jeep with the clutch out and between accelerating and decelerating. Maybe it's two problems.... sigh...

Sure enough, bolting on a new rear prop shaft solved the problem.

Man was it ever getting sloppy at the splines. The Replacement shaft seems beefier. I hope it holds up better.

A race car set up guy suggested a pair of two degree shims under the pumpkin to keep a bit more load on the U joints. When I installed the new shaft, it seemed slightly offset, so I'm thinking I'm going to forgo that idea. Anyone had M38 experience shimming the rear end to extend the shaft life?

I can't believe how smooth and quiet my Willys drive train is now!