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I want to update you and give a summary of the dreaded Modulating driveline vibration at highway speeds that I have been trying to resolve.

At first the vibration was slight and not too bothersome.
I replaced both leaf springs and bushings to correct ride height and restore comfort. That made the vibration worse!
I replaced the u-joints. That made the vibration more worse!
I messed with driveline angles with some positive results, but not totally fixed.
I moved forward to the transmission, I replaced the rear bushing. This bushing showed some big time wear!
I inspected the transmission yokes internal splines with a flashlight and noted deep wear at where the tailshaft ends inside the yoke. Once everything was reassembled.
The looseness at the yoke at the output of the transmission was reduced by 50%.

I remeasured my drivline angles and noted that I could remove those axel shims and return the rear end to its stock angular location.

I fueled the car and me and the wife went to a car show about 40 miles away and noted the best ride ive had in that car at 65 mph! Ever!

The ride back was not as good. Unsure if a lighter car due to fuel consumption? Maybe wear on the new bushing?
The vibration is back........

My thoughts are this:
Replace the transmission yoke? Or replace transmission yoke and driveshaft as a assembly?

My observation on the dreaded modulating drivline vibration is: Wear, Looseness, With slight misalignment.
Wear makes parts loose, Loose parts can move, If loose parts can move? Will they maintain there rotational centerline.
The answer is no.

What bushings are you running in the front eye and the rear shackles?

Also, when you put the springs into the shackles, did you have to do any prying to get them to fit?

One final one.. did you do any full throttle blasts before you go to the cruise night, or just as you were leaving?

As for your fuel consumption theory, try this. Put a couple bags of sand/rocks/etc in the trunk and drive around. See how it feels. If it smooths out, you know it's an angle issue (lowering the ass end is changing your driveshaft angle)

I'm running the stock rubber bushings in the springs. Everything assembled with ease. No full throttle blast was performed.
everything is new under there except the yoke and the driveshaft itself.
Going to look for a salvage yard driveshaft...Meanwhile I'll get it back up on jack stands.

I'd recommend taking your original driveshaft to a shop in your area that specializes in fabricating and balancing driveshafts and get their opinion on what's causing your problem before you continue spending money on items that may not be causing your problem.

Richard

I totally agree Tom, old used parts are old used parts and just because a used driveshaft worked well in it's original application, it might not be properly balanced correctly for another vehicle.

If the car is drivable and the shop you find isn't too far, I'd take the whole car to them so that they can test drive the car and get a good feel for what's causing the problem.

Richard

Yup, I would get some measrments and have a new one made up, go with some good u-joints.

I might have mentioned this before, but make sure the drive shaft shop can balance the shaft with the slip yoke on. Ask them, because I went crazy with one on my 4x4 van until I found a guy who can balance both pieces together.

Everything got worse when you changed the springs? Do you still have the old ones? Maybe you should compare them to the new ones carefully,

Since on the subject of driveline vibrations, Hot Rod this month had a good article on it. Granted it's for a Mustang, but the principals are the same across the board.

Made the call to get one made in Duluth Minnesota. 130 miles from my house. Lets see what happens.

Update:
As I stated above. I called the drive shaft folks and all was good on the phone. I brought them my shaft and my real world measurments. The guy was a JERK at the counter!
He said, Go home and keep looking for the real problem! I can take your money and build you a driveshaft and you will be pissed if it does not solve the problem!
I asked him if he could just check my shaft for balance and straigtness? He said sure! Next week sound okay?

Well I drove back home 130 miles........I said @#&^%$ and had my garage floor epoxied!
Today was the first day back on my new floor.
Anyway I bought a super nice dial indicator and measured the runout on the shaft in the car as assembled rotating the rear wheel. I zeroed my indicator and marked the shaft.
My readings were +10 thousands to a negitive -70 on the dial. a total of 40 thousands as I see it and its repeatable!.

Bent shaft?