Have you looked into some softer tires? 40 or so years ago I bought a truck which had some fairly new Atlas tires on it and at any speed over 50 they'd do the same thing. It was nice that the tread didn't wear very fast on them, but they would absolutely scare you to death if you had to stop at 65. You almost couldn't do it on a dry road and forget it on a wet road. I went to a tire dealer who changed them for some others (Goodyear) and it made a world of difference.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end