You did not say if you have a stock suspension or a lifted suspension. I had a similar problem when I'd hit the brakes. I decided to change the factory shocks since they had 115,000 on them. The new shocks fixed the jumping problem and revealed I had warped front rotors.
How old are your shocks? I recently saw a Monroe Billboard which said to change shocks and struts every 50,000 miles. My 1969 Camero owner's manual said to change shocks every 25,000 miles. So if your shocks are over either of those mileages I'd install a new set.
I have installed the blue monroe matics thean after 30,000 miles I changed to monroe reflex shocks. The Reflex are a better shock and plan to install another set when the time comes.
You also may want a front end inspecton done, you may have a worn ball joint or rod end bearing. You could even have a bad hub but doubt that. Time to go back to basics.
You may also be suffering from Death Wobble, do a search and see if what happening to you is that. Death wobble happens with lifted suspensions if I recall correctly. I never experienced it.