Vibration from 68--78 mph

[Schurkey]

Just solved my vibration problem...again.

2003 Trailblazer 4.2, 4WD. NOT extended version. Currently 247K miles.

Several years ago, I had a vibration and noise from 68--78 mph. No* shake at 65 or at 80 mph. Worst about 74 mph. Not dependent on power or trans gear--would do it on throttle, or coasting; in OD or D. 2WD or 4WD. Made no difference except for vehicle speed.

This was eventually traced to a faulty rear U-Joint on the rear drive shaft. I had the shaft serviced by the local specialist--they slapped in both U-joints then put the shaft on their lathe. They told me the shaft was slightly bent, but they straightened it--got "almost" all the bend out of it--and balanced it. Reinstalling the shaft, the vibration and noise was G-O-N-E. In fact, the vehicle was so smooth that It was actually better from 25 mph and faster--*I did not realize there was vibration at that low speed until it wasn't there any more!

So several years go by, and my vibration comes back. Same story--68--78 mph, floor shakes, seat shakes, steering wheel is smooth. Vibration is accompanied with a "booming" noise inside the vehicle. Just like before, it's worst about 74 mph. Everything was exactly like the first time.

Except the rear U-joint is fine. So is the front U-joint of the rear driveshaft.

Balancing the wheels doesn't change the vibration. Changing wheels entirely doesn't change the vibration. Rear axle does not seem to have wheel-bearing or differential problems.

I grabbed a rear driveshaft from a 2008 Trailblazer being parted-out. The rear U-joint of that shaft was predictably wiped-out, so I squeezed a new one in myself. Front U-joint is fine. Installed the replacement driveshaft.

Smooth as glass at all speeds. AGAIN, I didn't know I had vibration from 25 mph-faster until it was gone. And the horrible vibration and noise at 74 mph is gone, too.

APPARENTLY, the original driveshaft either went out-of-balance; or the bend came back.

APPARENTLY, the 4WD short-wheelbase GM360 is sensitive to rear driveshaft problems. The extended-wheelbase 4WD rear driveshaft is longer, of course--but it's also MUCH larger diameter than the short-wheelbase shaft. IF (big IF) this vibration ever comes back, I'm having an extended-wheelbase, large-diameter rear driveshaft cut down to the shorter length.

SWMBO is very happy that her ass and eardrums aren't getting massaged at highway speed any more.