Old Ford Death Wobble

Hi there,
So I have 1976 f100. For the last few months I have been dealing with the famous death wobble of solid axle trucks. It seems to randomly appear at speeds over 45mph or after hitting a speed bump or cattle guard. I have tried several things like replace all the bushings in the front. Ive looked for any bars with play and fixed them right away. And I even replaced the ball joints thinking it would help. I actually let another mechanic help me and though we solved the problem, I was fine for about 3 days. Im running out of parts to check and its hard to drive it down the road. Any ideas?

Have you had someone watch the front steering components while someone else saws the steering wheel back and forth? Worn steering box? Tie rods?