I've been chasing a harmonic vibration that seems to be located in my driveshaft for the past couple of weeks. It comes on around 60 mph. Both the tranny and the driveshaft have been given a clean bill of health but the mechanic commented that I seem to have quite a bit of backlash in my rear axle. Could this be causing my problem?

 

I would almost guarentee that is your problem. as long as it doesn't come in any sooner than 60, you should be just fine. It is when the hum comes in at 25 or 30 mph that you really need to worry. Mine hummed at 30 and in a month it was popping off teeth. The humming is not good don't get me wrong, but there is pretty much only one shot for these things. If you have driven on the hum for a while, you are almost stuck with it.

Who installed the rear, also do you have it full of fluid?

 

It's got fluid up to the filler hole. The posi and 3.73 gears were done by a local shop about 11 years ago. They've always had a little whine to them but have never given me any trouble until now. The fluid has always been changed every 15,000 miles with the posi additive.

 

11 years is probably a good run for the gears. I say this because I have had 2 installed for me and I installed 1 my self in less than a year.

Check the teeth out manually. You could have worn out a bearing too.

If not, a fresh set installed might be your only alternative.