Laughingly called "Death Wobble" most of us are familiar with it at one time or another. It is very common. It's caused by looseness of some part OR PARTS, in the front end. Note I added a plural. Thats because It can be more than just your kingpins or your steering balls.
The best way to find it is to jack one wheel at a time and shake it and see what's going on down there. In one of my cars, it was the bushings at the ends of the springs, but I found the bad kingpin first. One expensive cause is a bad steering box. How much play is in the steering wheel? Ideally there should almost none. These can be adjusted. There are four adjustments. Much has been written about them so I won't repeat it here. If you can't adjust the play away, the box must be rebuilt. Good Luck! P.S. Your steering balls on the arms should be COMPLETELY Round at 1.000 dia, If they're out about a 1/32, file them back to round within .020. and start looking for replacements. More than that,and the balls must be drilled out and replaced.
Terry