I have a 1969 mustang and when i get up to 55-60mph the whole car shakes. I have new tires/wheels had them rebalanced, new drive shaft, rear axle is straight. What could be causing this problem?

 

front end alignment?

 

no i just had a front end alignment at the ford dealership

 

Balance issue. I'd start with getting your tires balanced.

 

he had them rebalanced already.
does it happen downhill with no gear as well.
as in engine speed at idle but car drivin around 60mph ?

pinion angle ?

 

Checked your wheel bearings? I had a similar issue early in the summer, bad vibration but only at a certain speed. Took the wheels off, popped out the bearings to find them littered with metal flakes. Simple replacement and the problem was solved.

 

kalli it only shakes when its in gear.
brand new wheel bearings where put in during restoration.

 

if it only shakes when in gear then the problem should not be driveshaft and from there back (axle, bearing wheels ....)
check the following:
drive 55-60 and then press the clutch and leave engine in idle. sounds like you tested this already and it doesn't shake then
then do the same thing but have the car reviing at 3500-4000 with clutch pressed (try wioth car standing as well). Dos that shake the same way ?
If yes the first thing I'd check is if something is hitting the chassis that shouldn't. As in gearbox, headers, etc no part of the engine and gearbox should touch the chassis anywhere.

 

My bet is driveshaft or U joints. If you put it on stands so you can see the driveshaft rotate, you'll likely be able to see the warp. Any small amount out of perfect alignment will translate through the car just as you described.

 

I had the same problem on the highway. when i put posi and 4.11s the shaking were reduced drastically.