Remove the door panel. Check for a broken spring ( clock spring) on the door unlocking mechanism near the square pin that attaches to the inner door unlatch handle. If thats OK then check and make sure it's connected to the door latch. Go to the rear of the door and look into the small square hole where the unlatch mechanism arm attaches to the door latch.

Jason, I have found more than once, the screw where it pivots on the back doors falls out, subsequently doesn't allow the latch to actuate. Either way, as Lance has described, you must remove the door panel to diagnose.