Shaking at highway speeds.

Check wheel balance.

you mean to say the engine is missing?

pull out spark plugs and see if they all look the same,could be dirty fuel injectors or a blocked fuel filter

run the fault codes and look for problem cam or crank position sensor
could be a problem engine mount that is being started by a misfire

bad/ or out of ballance tires or a bent wheel

I have this same car with exactly the same problem. Begins shaking at 40mph and stops shaking as soon as you let off the gas, but then begins again as soon as you press on the gas and smooths out as you get over 50. And the shaking is not felt through the steering wheel. Solution: Replace the transmission mounts. There are two of them, one on each side mounted on the frame right next to the shock strut. There is also a front and rear engine mount, but I replaced the two as described and shaking is gone.

U-joints.....seems like your vehicle wants to launch off the road?

rack and pinion is the steering wheel joint, so if the steering wheel shakes or handling has decreased, then i would say it is the rp. , also check the wear on the tires for uneven wear, it could be alignment, does the steering pull to one side? it could be motor mounts just find the bolts and check em wiht a torque wrench, also inspect mounting bushings as well. to check trans. give it alot of gas to get past 35-45 mph and the shaking should go away at higher speeds, if it doesnt then its the trans missing "third".

Have you checked the U-joints in the rear drive shaft? Also did this happen before the tires were replaced? Take the truck back to where you got the tires and have them check the balance, they should do this for free.