I'm getting ready for a long trip so I'm trying to get everything right with the brick. My car has a noticable vibration on the highway. I can start to feel it between 45-55 mph. It seems to disappear around 60 and everything is smooth until I get a little over 70 mph. From 75-85 its pretty bad. My seat is vibrating and the car has a little bit of a boom to it. You can actually hear it vibrating. What could be causing this?

Here are the things I've eliminated so far:
1. Bent wheels - the car had bent wheels when I got it but these have been replaced with a straight set.
2. Carrier bearing and support - both new.
3. Transmission output shaft bushing - no play and no visible fluid leak.
4. U-joints - checked all of these out when I replaced the carrier bearing.
5. Tire under/overinflation - Experimented with this last night. I couldn't tell a difference as I varied the pressure.
6. Wheel balance - I will have this done after work today.

I appreciate any responses. I love driving this car on the highway because its so comfortable and normally smooth. Right now, I'm dreading this trip because I'm afraid I'm going to be shaken to death by the time I get home.

Thanks
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Don Tibbetts; Living in the Florida Panhandle - soon to be known as Disney World North - The St. Joe Kingdom; 92 745T 95K mi