Well, I took the Jeep in to a local shop to check the spacers and they were still torqued all the way to when first installed. So they threw the tires on the road force balacing machine, and it turns out one of my stock front tires wasn't perfectly round and was most likely the cause of vibrations. They said it failed the road force balance because of it. I just don't understand why I'd feel it now, immediately after spacer installation, and not before - during the other 4000 miles on them...
They offered to contact Goodyear for me tomorrow to check if I may be able to get refund credit towards a new set under warranty, considering they had been used so little and the imperfect tire. I obliged, as I'd much rather deal with them before trying to take it up at the dealership, if it comes to that. I'm interested to see what, if anything, Goodyear is willing to do.
I guess we'll see how it plays out.
BLOO > Traded in for '16 4runner
Thanks for all the info about Heeps from the community!