Bias ply tire shake but sometimes smooth as can be
I have a 93 Toyota peekup with Super Swamper Thornturd tires 35-14.50x15 on 12" wide Weld wheels. One tire has a problem, sometimes it'll run down the highway as smooth as can be and others it'll shake bad enough to make me slow down. I can watch it in the mirror and when it is shaking, I can actually see the tires appear to be moving in and out like the tread is squirming. I've moved it to the front and the shake moves with it. Fastest I run the thing is 60MPH but this is frustrating over a 20 mile drive one way to work. I keep the tires at 20 PSI I've run them up to 30PSI and as low as 10 PSI and this is the only one that has the problem. The tires are about four years old and I've never reversed rotation on them, I rotated them front to back and that's it. I remember an old wive's tale about do not make a bias ply turn backwards to what it has alwys run. Any ideas?