That's nuts, the two systems aren't even related, I would be looking for some rubbed through wiring or a cross circuit from the 4X4 indicator light to brake light wiring, possibly behind the dash. Do you have wiring diagrams of the vehicle? if not it might be a good idea to get them as I think you in for some circuit tracing.
Search Chevrolet C/K 2500 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Chevrolet C/K 2500 Experts