This may be a very silly question, but when upgrading the wheel from 16 to 17 " and getting bigger tites (285/75), does the spare wheel still fit into its original space? or will it need to be mounted behind?
Yes, but with most hitch receivers, it will no longer sit flat against the carrier. Instead, it will rest on the forward tabs of the carrier, tilting the tire up towards the rear. I don't see any problem with it sitting like this, other than more debris may be thrown up onto the tire.