90 Scottsdale long bed "shimmy"

[Vicb]

I have a 90 Scottsdale with 53K. Just replaced Uniroyal Laredo's with Douglas P23575R15's. I have a shimmy between 43-52mph that comes and goes. Some days its there, some its not. Checked air pressure, balanced the new tires twice. Used a Road Force balancer one time...they said one tire was bad. Took it back to the dealer, they rebalanced, said it was fine. Shocks don't give the bad mushy bounce when I jump on the bumper, but they are original. Front end has been aligned. I feel darn near every road bump as if I'm riding on tires made of wood. I could run over a pea gravel and feel it. Anybody got any ideas?