1999 Jeep Cherokee front end shaking

I recently had state inspection (Pa.) and it passed. A few days later while driving the front started shaking violently. I slowed down below 50 mph and it stopped shaking. When I got home I rotated the tires and the shaking stoped. My steering wheel was off level. Thinking I needed ball joints or tie rod ens. I got a 4 wheel alignment and had the steering dampner replaced. The front wheels still didn't feel right. The other day going 70 I hit a bump and the front wheels started the violent shaking when I pulled off and slowed down and got back on doing 70 it never happened again. When I hit small bumps I still feel the wheels shake a little but it stops while driving at 70 mph.