shakes at highway speed

Hello,
I have a 73 Swinger that sometimes shakes real bad at highway speeds. If the road is smooth the ride feels fine, but if the road is a little rough the card shakes a lot. I mean I thought my tach was going to fall out of it's mounting. I drive this stretch of freeway almost everyday in another car and you can feel the roads a little rough, but it doesn't feel like the car is going to shake apart like my Dart.
All the suspension has been rebuilt except for the steering gear. The steering gear does have a fair amount of slop. Would the steering gear cause this? What else should I look at?
Thanks