Tolerance
Check the tolerances per the manual specifications.
If it's too far out of tolerance, you'll need a new interlock sleeve (very rare), or you'll have to implement the workaround Willys calls for in the Maintenance Manual.
Check the shift action (this will confirm the tolerances):
Install the outer levers.
With either lever in a "gear engaged" position, try to shift the other lever. The interlock sleeve should prevent double gear selection. The other lever should barely move. If it rotates too much, you'll need to deal with the interlock issue.