With the spacers and the farther out the wheels are any looseness in steering becomes amplified.This is why I had the steering shake at 60 to 70 mph range as well. I wasn't running spacers but my 22" wheel offset is +15 . I only had the shake with the +15 offset rims and not with the stock rims. Tightening the steering rack adjuster nut fixed the shake issue.
https://www.infinitiscene.com/search/1/?q=11604-Steering-
Over two years I went through having a wheel straightened,Road Force balance, Allignment and still the 50 to 60mph shake persisted. I thought it was my 22" rims because when I ran the stock wheels the shake was gone. But what was really happening was that as the wheel gets wider and the offset is further out any looseness in the steering system is amplified. I was so amazed at how easy this fix was after all this time. A lot of others took theirs into the dealer and they were not able to fix the shake issue. One of the best DIY's ever