diagnosing vibration

[gcam]

Okay, 1996, 4.0, np231, auto, chrysler 8.25 rear axle. No lift.

I get a vibration between 60 and 70 mph. It peaks in intensity at about 65, and wanes as I reach 70. Beyond 70, it isn't there. Below 55 it isn't there.

I've rebuilt both driveshafts (it was getting time anyway, so did it regardless). I can't find any dents or bangs in the driveshafts, and no evidence of missing weights, either. there is no change since I rebuilt the driveshafts.

The vibration seems to be speed dependent, and not rpm dependent, because it shows up at those speeds whether I'm in drive or OD.

There is no feedback through the steering wheel, and I can feel the vibration on the floor, so my first instinct is that it is something in the powertrain.

I've even tried driving around without the front driveshaft for a few days, and the vibration was still there.

The Jeep is completely stock, and I don't use it to tow or haul anything. Merely a family vehicle.

Any ideas? Thanks.

Geoff C.

[8Mud]

The first thing I´d do is put the front tires on the rear and the rears on the front. Cheap test and you probably need a rotation anyway.
A cheap test for direshaft balance is to fill the Jeep up with a full passenger load (invite the nieghbor kids for a drive). Changing the driveshaft angles often changes the vibration. It doesn´t have to be dings or dents. Rust or undercoating can mess with the balance (but rare). The transfer bushing is an overlooked part that has given me trouble in the past (usually slings some oil at the rear seal).

[gr8tires]

I'd say it's more in the tire/wheel unit. How many miles on tires? What size, and more importantly what rim width? It's possible you threw a wheel weight along the way, like we did off-roading on mild roads. We had same problem(after scraping the mud/clay off the wheels) we rebalanced the tires, and No Vibration.
Good luck.

[sidriptide]

i am also thinking its tire balance related... rotating is a good idea... equal pressure... even an out-of-round tire can be balanced on a spin-balancer.. but still wont run smoothly.. if you still think it's driveline.. remove the rear d/shaft(plug the output seal of course) and drive it in 4HI front-drive to see if its still there... you didnt really state if this is a new problem or finally trying to figure out something thats been going on for a while..
mike