Originally Posted by wlbuckles
Guys, I just had the rear end rebuilt and a new tranny installed. Took it out for a test run a discovered a pretty good vibration beginning at about 40 mph in fourth gear. I'm thinking maybe having the driveshaft balanced ?? Any thoughts on this would be greatly appreciated. Thanks.
Check the u joints on the axle end of the shaft and make sure they're fully seated into the cups on the yoke and centered. Also make sure that the bearing cups on the ends of all 4 parts of the joint cross are of the same type - either "innies" (snap rings on the inside edges of the cups) or "outies" (snap rings on the outsides of the cups). If that's not it, then the next thing to check is drive line angle (sometimes referred to as pinion angle but that's somewhat misleading with regard to what you're actually doing).
That's my prime suspect since you said "new tranny". You have to make sure that the centerline through the crankshaft/transmission runs exacly parallel to the centerline through the rear axle pinion, otherwise you'll get a vibration like the one you're describing. Please notice I said "parallel to", not "directly in line with". You don't want the centerlines in a straight line with each other because that will stop the u joints from moving at all and they'll eventually fail due to brinelling on the cross ends. Here's a link to a graphic that shows what you're trying to acheive: http://www.4x4wire.com/tech/pinionangle/stdgd.gif
, and another description here: http://www.markwilliams.com/driveshafttech.aspx
Keep in mind that the lines have to be parallel in both directions: both "up and down" (as viewed from the side) and "side to side" (as viewed from the top).
You'll need an angle finder to measure it - there are several types - here's one
. If yours is "off", chances are that the wrong rear transmission mount was used or there's something else "different" about the tail of the tranny/crossmember. Correcting the angle if it's off will require either changing/modifying some of those parts, or installing a set of adjustable UPPER rear control arms - like these perhaps.