You have an internaly splined hub. Those came in newer 70's trucks. 60 truck should have a external hub that is held on by bolting to the spindle.
I have has problems in the past with the springs in your type of hubs. First off have you used grease in the hub? They are not susposto use grease and many won't engage/disengage with alot of grease in there. Secondly try bending the springs to they eather engage or disengege more...witch ever is not working. Over time the springs fatigue and loose their arch.
