Vibration at High Speeds

[scooter3]

So I've got my car to the point I am taking some trips on the interstate and when its getting to around 65-70ish I am getting a vibration. It seems to happen worse when I let off the gas and at times under throttle its constant and sometimes it goes and comes like harmonic vibration. It feels and sounds as if its coming from the rear but its a bit hard to tell.
Two Key Points:
1. The car is lowered 2 inches in the rear and the drive shaft angle is not great.
2. I have already replaced the rear u-joint because it was clunking from the slack. After I replaced said u-joint I started having this problem.

Now my rear wheel bearings/seals do leak a bit, but nothing terrible like pools of oil at my wheel or in the drums. Its more like a seeping and the diff is full of fluid.

Anyone have any idea of what this could be?

 

[Big_John]

2. I have already replaced the rear u-joint because it was clunking from the slack. After I replaced said u-joint I started having this problem.

You have almost solved the problem. Something is wrong with the U-joint installation, U-joint or driveshaft.

Front or rear u-joint? Are the clips in right? All the balance weights still on?

If you can't find the issue, take the shaft to a shop that makes driveshafts and have them check and balance it.

 

[65sporty]

I agree with Big_John. I would also ck the pinion angle like you mentioned

 

[scooter3]

You have almost solved the problem. Something is wrong with the U-joint installation, U-joint or driveshaft.

Front or rear u-joint? Are the clips in right? All the balance weights still on?
I replaced the rear u-joint. I put it back on the same way the old one came off. Maybe there is something I missed but it was pretty cut and dry.
When I had the driveshaft out it had the weights on it but I plan on having it balanced after this problem came about.

I drove it more this week and it vibrates pretty hard at around 70 on and off throttle no matter what I do.

 

[Big_John]

At that speed, it could be tires or driveshaft. Since you messed with the driveshaft and then the problem started, well... it seems logical that it's the driveshaft.

Driveshaft could be out of balance, bent, twisted or maybe as simple as a bad front u-joint.