I have a '98 F-150 (4.6 L) truck with approx 166K miles, and lately noticed a slight shimmy at >55 Mph. I have new balanced tires, and have replaced the upper and lower ball joints about 55K miles ago. Is there any checks that I can do to find out what it is, steering linkage, CV joints, idler arm? How does a shop check and identify the problem?
Any info is appreciated.

 

New balanced tires doesn't always mean the tires or wheels don't still have issues. I'd get a roadforce balance first and also run your wheels and tires in a balancer to visitially inspect for any slight warping of the rim or an out of round tire.

Where did you get them balanced? If it was a chain, I'd suspect they could still be way off.

 

If....if your sure that all the other causes have been addressed, look into the possibility of a 'warped' front brake disc.
This condition often come and goes in severity according to brake usage and disc temperature.
What often happens is this comes from tightening the wheel 'to' tight such that it takes the disc into physical distortion.
Then heat takes over to change and set the condition as permeant.
A 'wave' in the rotor must pass through the caliper pads and cause some friction at every revolution, setting up the shimmy .
Good luck.