I had the same exact issue. I fixed it 100 percent by buying the daizen steering rack bushings from Amazon (my original bushing washers were ovalled out from the vibrations), and replacing the LCAs with the OEM sub-assemblies for the IS. Quick search will give you part numbers for all of these. The LCAs were the culprits, and the steering rack bushings were ruined by the vibrations. Changing both completely fixed it.

 

I have the same issue as well with my UL and I just ordered those daizen bushings and factory aluminum LCAs. I had a bent wheel and bad DS front air strut that shook the front end apart. I replaced those to still have the shimmy over bumps. I can see play in those itemsI ordered but not in the BJ's or anywhere else. Hopefully they fix it. Part number info is priceless! THX fellas!

 

Chiming in as I am experiencing the shimmy as well. I have replaced everything in the front end besides the upper control arms, I am needing to as the ball joint in the passengers front UCA is getting clunky. The one thing that I have seen no one do in this thread is replace the tension bearing in the steering rack or play around with alignment. As well, is everyone torqing their suspension correctly? Suspension and steering members need to be torqued under load. I sure have. Given the current state of affairs in the US and being in a position where I can park my car for a week at a time, having access to another, I have noticed that when my GS has been sitting on the much lower friction garage floor for longer than a week, and I go to drive the vehicle, my shimmy happens every time I apply the brakes, until I start driving the car a little bit harder and put more load through the suspension. What I believe is happeng is that the load being applied through the suspension on rather high friction surfaces is relaxing when the car sits on the lower friction garage floor. Whilst driving, the feel through the steering wheel inclines me to think that my shimmy is due to the worn tension bearing in the rack. Why? The past two times I have gotten an alignment, my passenger front has toe has been out of spec, aligning brings it back. BUT when there is play in the rack due to slop in the tension bearing, we have a moving variable that I believe is causing a lot of the issue. I think we also need to start experiencing with toe angles, but this can only be done on a vehicle that has a front end that is completely in good shape.

Sorry guys, I thought I was posting this in the 2GS section, hope it still helps.

 

I've had my 98 GS4 for over 13 years now. Needed tires when I first bought it and when I got it back it had a terrible vibration in the front. Re-torqued the wheels and it went away. Now I always ask them to torque them in increments rather than just checking they are at least 76# after over-torquing them with an impact.
Also, nobody has mentioned checking the driveshaft for balance or true.

 

Just following up on my issue. After successfully having no shimmy for a month, It is fixed! After replacing the old air struts with BC coilovers my shimmy is GONE.

 

How do you like the BC's now that some time has passed?