No, the timing of the actuator motor and the shift fork in the diff must be properly timed for everything to get along. The front diff has no effect on the operation of the rear locker, other than if the 4wd indicator on the dash isn't illuminated steadily (which only happens once the front diff has engaged via the vacuum actuator) then the rear elocker won't be activated.
Basically, if you can get it into 4wd and the light on the dash is steady, then you problem is limited to the rear diff, diff actuator, or rear locker ECU. I'd suspect a mechanical problem in the rear locker way before I went chasing electrical gremlins...
Ryan
