I have found that out of balance tires cause a vibration around 50 mph and you can out run the vibration if you speed up. You might check that the rear u-joint is indexed into the yoke properly also. Had that problem in my race car.
Glad that CalBAR is involved. Those folks are relentless. When I worked in a transmission shop we had a complaint filed against us. Fortunately we had all of our ducks in a row. The customer was being unreasonable. As said above, thank you for your service. I hope this all works out for you.
65s, I'm glad you chose not to use that thick vinyl. My son's Dart had that on it it, when I pulled it off last fall it was soaking wet underneath. It hadn't rained here in months. That stuff makes for a nice wet blanket but not a good top.
There was a guy who used a rear end out of an Explorer. I don't remember if the thread was on here or another board. The guy said it was an easy install.
Got the kid's 340 up and running. He rebuilt the motor this summer for his senior project. The car runs good, but the intake is leaking oil. I guess I know what we'll be doing next weekend. :)