Have a helper turn the steering wheel quickly between the ten and two o'clock positions while you watch the linkages. Look for movement between any ball and socket on the links you didn't replace. If your truck uses an idler arm, watch the end to see if that arm moves up and down. That will let the right wheel move left and right. It's common on GM vehicles for them to be so bad that you can move it by hand and see the wheel turn back and forth. Don't overlook the pitman arm on the steering gearbox, and a worn joint in the steering shaft going into the gearbox.
If the looseness you're referring to means it wanders on the road and you have to keep on correcting the steering, "caster" can be too low. That is one of the three main alignment angles and is largely responsible for the steering coming back to center after you go around a corner then let go of the wheel. Higher caster makes for harder steering but much more directional stability. We added power steering in the '60s to make up for that harder steering. Caster is difficult to change if you have the I-beam suspension. It's easy to adjust if you have the upper and lower control arms that every other manufacturer used. Too much caster is bad though too. That can lead to overshooting when the steering wheel comes back to center, and a severe shaking back and forth until the vehicle slows down.
If the looseness is noticed only at highway speeds, the alignment is suspect. Did you have it aligned after these parts were replaced? If the two front wheels are steering in different directions, the truck will follow the one with the most weight on it, and that will change with bumps and slants in the road. How are the front tires wearing? Anyone at a tire and alignment shop can inspect the steering and suspension parts, and they can show you how to "read" the tire wear.
Steering gearboxes do not have to be replaced when they develop play. There are adjustments on them for that. If you installed a used gearbox, you may have installed the same problem.