I just bought a GQ Jap import safari. When I put it into 4wd the rear diff locks up automatically. There is no vacuum actuator on the diff housing and no diff button in the interior like my other GQ had. The rear diff unlocks upon reengaging 2wd. It has no aftermarket diff locks installed.

I want to be able to isolate this from happening as I dont want it always locked in 4wd - any ideas?