You said the tires/wheels were balanced and road forced, but what were the road force numbers (in lbs for each corner)? Just because they were road forced doesn't mean there isn't a bad wheel/tire in the mix. For instance, a wheel tire could show a "passing" result with 19lbs of road force, but I've found that anything more than 9lbs of road force will cause vibration on these cars. It's a huge pain trying to find tires under that threshold, but in the end it's the only way to get as close to zero vibration as possible IMO.
Not ruling out a bushing, bearing, hub, etc. in this case though...just trying to remove as many variables as possible in hopes of narrowing down the problem.