[67fstbk]

New transmission, new driveshaft, new u-joints, new ring and pinion, tried other wheels and tires put on it for a test and I still have a vibration. Mine starts around 50 and starts to go away around 60. And it's not a little vibration like you get with an unbalanced tire. It can be teeth chattering.

[69moneypit]

It sure could be that the rubber insulator of the trans mount is torn away from its metal attachment as was mine on my 69. Cheap fix if that is it. Tell us, when did this start happening? Did you replace a rear end, drive line, transmission, u-joints, or torque converter before this started or has it always done it? I have a 70 with FMX. I had the trans rebuilt, new trans mount, drive line and new joints balanced twice, axle bearing replaced and it still does it. Only thing i havent done is replace the torque converter, check the harmonic balancer, and check the rear end bearings and gear mesh. I bought the car in a basket so i dont know what it did before i got it. I definately feel your pain.

[desdain]

Okay, update:

After having checked the axle shafts (again) and also checking the play in the differential, we noticed that the shaft splines had some play in the differential, maybe a little less than a millimeter. Though this was a point of concern, I didn't have much choice than to re-assemble everything and come to terms with the fact that the vibration might never be fixed.

That is until we clamped a hose clamp with a small weight to the drive shaft (read in this or some other mustang forum that a guy had done this). We measured the "wobble" in the shaft which was about 1 mm, and then added the weight on the "low", or inside, side (don't know how else to explain it). Took it out for a test drive and the vibration is almost gone, just a slight hum at 60mph instead of the earsplitting, brain melting drone that I had originally. So now I have found a place in town that can balance driveshafts (not too common around here), so hopefully the problem will be permanently solved.

Thanks to everyone for all your tips and help. I really do appreciate these forums :-D

[desdain]

Just a final update.

Had the driveshaft highspeed balance and made sure that the phase was correct when re-installing it. There is barely a sound from the drive system anymore. Unfortunately, with the drone of the un-balanced driveshaft gone, I could now here the ear piercing whine of the differential. Long story short, we added 770um (micro meter) of shims to set the hackspacing and adjusted the backlash so that median gear had 250um of movement. I call it median because there has either been some wear on the gears or Yukon sent me a crappy gear so there was some variation between the teeth of the gear from 150um to 320um.

Now it's really quiet and I'm just adding STP VibroPlast to get rid of the road noise. Soon it'll be quieter than a Lincoln ;-)