Vibration at 32-35mph

What could be causing this shaking of my car at this particular speed range? It is quite apparent in the steering wheel and the seat, occurs when I’m accelerating and decelerating, driving uphill and downhill. Remedies I have tried: new tires, rebalancing the tires, checking tires for too much run out, new shocks, replaced rear end assembly, replaced rear axle shaft, cv joints, and rear wheel bearings, replaced tie rod linkage, and had the flex discs checked for cracks.

My car is a 1985 Mercedes 300 TD wagon.

Thanks for your time. Hope I haven’t confused you.