As Broncobowsher stated, complete disassembly required to remove two rods between shift rails.
The two rods prevent front drive shift rail from moving before the rear drive is engaged.
Below picture shows shift rails in neutral.
Pulling back on T handle shifter pulls rear drive shift rail to the left (in picture), engage rear H drive.
When rear rail is stopped by detent ball, the righthand rod can slide into relief in rear rail, allowing T handle shifter to pull front rail to left engaging front H drive.
Pushing on T handle shifter accomplishes similar results with lefthand rod and L range drive.