Solved: I was using the wrong Animation blueprint. I had two in my project, and I was attaching the 1st ABP to the new Pawn I was using. As soon as I switched it out to the current ABP, it worked fine. Completely missed it. Wow. A whole weekend wasted. 
(I think the initial issue was that the model I imported had the caliper bones parented to the wheel bones, so even though I separated them inside UE, for some reason it didn’t work. I reimported a different model where the two bones were properly separate, but then forgot to switch out the old ABP for the new one.)