Kenworth dealerships used to have a machine called " vischeck". It was a in shop simulator operated by a mechanic that was under the truck while the on road motions, wheels being spun, everything, was being performed. Palmer trucks in Fort Wayne had one, as well as Hissong Kenworth in Richfield, Ohio. It was $130.00 back in the early 2000's, but so well worth it. Problems that nobody could find the cause to were detected with the vischeck. I remember a driver brought his truck there thst had a "clunk" that could not be found, as everyone was throwing parts on it. They put it on this machine and found it to be the bad cab mount, left front.
But once I had the same problem as you, and it ended up being a lost front brake drum weight. But I do know that they changed from adding weights on drums to balance them to "gouging" the drums with a grinder, so to speak, to balance them, and to resolve the problem of losing drum weights for many years now.