Shakes left right on 50-60 mph and jumps slightly on 30-40 mph.

The jumping around I would assume shocks. Guessing they are past their life.

Any odd tire wear? If yes then possible your tires are reason for bounce at a certain speed.

The shimmy/side to side movement most likely come from worn components. First thought are tie rod ends.

Any play amongst the components? Does it go away when front end gets greased? Any grease fittings not taking grease?

 

That's a good answer.

You need to go over your whole front end.

Kingpins, tie rods, spring pin bushings, wheel bearings, tire balance, all of it.

 

Have any noise when this happens?

 

Also change the shocks under your sleeper if you never have. But check everything on the front end and also make sure your brake drums and wheels are centered.

 

Have a good alignment done... Its crazy how just a slight alignment issue can cause shakes and vibration.

 

Out of round tires will make that thing wobble like a Weeble, too.

 

My shakes ended up being the kingpins. Find a shop that specializes in suspension.

 

Op
When did the problem begin?

 

Wow. A lot of info herr