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

Could be #### near anything. Front end parts, wheel bearings, steering box, draglink, driveline, cracked wheel or hub, bad drum, torque rods on back, bad tires, bent rim, rears out of square. Youre gonna need some good jackstands and jack and a pair of coveralls. Use a monster prybar and a partner to see what you can wiggle. Could be multiple parts.

 

How many miles?

Look at the cab mount bushings. I know on my truck the ride was pretty bad until I had them R/R'd

 

I had this problem , when I installed new steers it went away. Be fore the new steers I had a bad shake at 45 to 55 mirrors would shake like hell. Put on a set of Centrimatics and smooth as silk. Ymmv .