The 83 wagon has a vibration that begins about 60 mph or so. It goes away at about 70. It happens most of the time but sometimes doesn't begin until slowing down from higher speeds.
My brake and tire store has balanced the tires several times. I had a warped wheel on the right rear but got it replaced today and it didn't help.
The tranny shop can't find the cause.
Take out your front driveshaft temporarily. If the vibe is gone, it's either that or the front diff/axles with too much wear. If not, it's possibly a front/rear wheel bearing going out, converter lockup going bad (it'll get worse and then at lower speeds till it suddenly stops moving and spews ATF)
Do you feel it in the steering wheel? Check for play in the rear driveshaft? Is the pinion seal leaking?
When is the last time you replaced the front steering stabilizer? The harmonic is worse right around 60. Have you experienced a bad steering stabilizer yet? Its extremely voilent. They are cheap, so its good policy to replace them often.
I might also suggest having your rear driveline checked, you may have lost a weigh. That "Could" cause a vibration, not sure if it would be that bad but it's possible I suppose.