Driveline vibration - can anyone help?

[Xman]

My GTO (Monaro) has 22K miles and has developed a driveline vibration while cruising on the highway. It occurs between 60 and 70 mph in steady state and decelerating conditions. During acceleration it goes away. 4th, 5th and 6th gear has it. Clutch in and it goes away. Had the new tires rebalanced to ensure that was not the problem. It has become more and more noticeable to the point I'm going to need to get this fixed. Any suggestions of what this might be? I suspect it may be the rubber bushings at each end of the driveshaft. GM has no replacement parts here in the USA. Looking for alternative sources. If this is the problem are these bushings readily available down under? in polyurethane? Thanks in advanced!

[drif6]

I doubt it is the driveshaft bushings, as the driveshaft will be spinning the same whether the clutch is in or out, so disengaging the clutch should'nt affect the vibration. Normally I would then point to the transmission, but if it is still happening in 4th, then it can't be that, 'cause the trans is doing nothing (straight through the 'box) in 4th gear.

Have you recently made any mods? Are the tyres or wheels new? New exhaust fitted? I would start by checking all of the bushes in the powertrian and driveline (engine mounts, transmission mounts, driveshaft centre mount, diff mounts, rear subframe mounts), then check clearances to the exhaust.

Good Luck!

[shaness8]

Mine did the same thing, but only in 6th
A new gearbox fixed it, hope your car is still under warranty.

[Xman]

I'm in luck as the car is still in warantee. I also was relieved when the service manager drove it and after being at speed on the highway for 15 seconds stated "this would drive me nuts". So they are dedicated to fix it. Parts availability appears to be my challenge. In tracing down issues like this, I often like to start simple and cheap before I do big and expensive. That's why I was trying to track down the rubber bushings.