I have a 69 chevelle and have a vibration at around 30mph. I changed the driveshaft last year and it was okay, and has resurfaced again.  I have just had both universals changed and the vibration is still there, the weights are still on the driveshaft and its clean with no obvious trauma. The shop said the driveshaft looked in good shap. I?ve noticed that when the universal fits into the yoke on the rearend its slightly smaller than the yoke in width, the universal measured about 3 3/16" and the yoke was 1/16" wider. I noticed when I installed it I was able to slide the universal from side to side in the yoke. I have checked 2 other universals I have sitting around and they all measure the same width. Is there to be this movement from side to side in the universal while secured in the yoke?

[Message edited by Jason1 on Jul 22, 2007 at 11:44 AM]

The yoke on the differential should have no play in the cups where the u-joint sits. Over time the little ears on the sides of the yoke can wear out torque being applied to it. 1/16 of play shouldn't be enough to cause a vibration at such low road speed though. I'd venture to say that the driveshaft suffered damage around the middle and is creating a whip affect (as little as .030 out in the centre of a shaft is very noticeable). This is also magnified if you have swiched to a lower ring and pinion ratio ie. 4.11, 4.56 ect. Take the driveshaft to a local driveshaft shop and have them check for low speed vibration and measure runout. If the problem persists jack the car up and check the runout yourself under the car with a dial indicator.