I had a similar problem with a mid 70s pick up
You could carefully drive it up to 35 mph but hit a mild rough spot in road or just look at it wrong and suddenly a lot of wobble and oscillation
Going faster 44-55 mph it would calm down and stop
But a critical speed range was terrible
Turns out it was a combination of a soft wall bad tire and the shock absorber not sufficiently damped and the outside of the rubber tire would get out of round with the rim center and wobble at the resonance of the suspension and shock
I put new shocks on and better tires and never had a nicer ride afterwards
-s
