1950 f1 front end wobble

[35 coupe]

Took my truck for a ride this morning, and driving about 35-40 hit a raised manhole cover,1- 1 1/2" high with the driver's side front tire and it went into a death wobble. I had to pull over to stop it. It always ran good, the king pins and front seem good and tight, tires are bias 16" firestones, 40 lb air, no shake or vibs at 65mph. This is the first time for this. Any suggestions as to what causes this besides not hitting manhole covers.... THANKS