I have a similar problem but mine does it between 90 - 100mph. I've had 3 different sets of tires on my truck so I'm sure it isn't the tires causing the problem. I think it is being caused by pinion angle from installing the 2 inch drop shackles. I rarely drive that fast so it hasn't been much of an issue for me. My truck runs 90mph in the 1/4, if I do any other mods I'll do whatever it takes to get rid of the vibration.

 

come to think of it I have a 2/4 drop on my truck so mine may very well be pinion angle as well. how do you check and correct pinion angle?

 

Some 2/4 drops come with shims to adjust pinion angle after the drop is install. They would have been some little wedges that go between leaf springs and axle. Here is a link from DJM regarding Pinion Shim Kits: http://www.stylintrucks.com/Articles...s/article.aspx

I've watched a friend adjust pinion angle on Trans AM. He used an angle finder, took readings from 2 different locations (Drive Shaft and where the Torque Arm mounts) used a formula to get his current pinion angle then adjusted the suspension until he got the angle he wanted. I think for normal driving you want 0 degrees and drag racing between -2 to -4 degrees. This is just what I remember him doing/saying but should give you a general idea to set your pinion angle.

If you need additional information PM me and I will get you contact information for one of the best Chassis/Suspenion guys around. He is well known on the forums for F-Body's and Mustangs.

Hope this helps.

 

ive got a vibration at 70ish.. goin today to get my tires checked... and i also have a good vibration at WOT, which im sure is my Pinion angle... after my last broked output shaft it bent the crossmember that the center support bolts to.. we straightened it best we could.

 

Do you have stock or aftermarket wheels on your ride. if you have aftermarket wheels on your ride you might want to check to make sure the wheel SIT!! on the HUB if there is a gap between your hub and wheel then you will need to get some hub rings. you can get some at any good wheel and tire shop(plastic or metal)- make sure that they are a very tight fit!! This will eliminate some of the vibration. I have an 02 chevy lifted 1500 2wd and i get pretty bad vibration when i am driving on the freeway between 65-80 mph but it depends on which lane i am in. one of these days when i get a chance i will check it out.

 

You know what's funny, my truck used to vibrate between 60-65ish when it was at stock height. I removed the 4x4 lift blocks and installed 2" shackles (expecting that the vibration would get worse), and it went away. Truck is smooth as butter (when the transmission works lol). I guess that my pinion angle was messed up from the start...I had the original vibration since the truck was new.

 

I will have to find away to check my angle I know its off but I have no idea how much. Do you know if you can buy shims at a local parts house?

 

You should be able to get an angle finder from a local part house for $10 or less. I don't konw if part houses carry the shims or not, but you should be able to order them from anyone that sells lowering kits. Here is a link regarding pinion angle:
http://www.wolferacecraft.com/pinionangle.aspx