your truck may be equipped with a chain slack switch.
there may be a switch on one end of the chain anchors that is open (or wiring is open) which will cause no lower (wants you to lift to take the slack of the chains up so switch will close. travel disabled to prevent carraige from slamming down if it was hung up on racking or something. as well as no lower to prevent further hang-up)

if your truck does not have chain slack switches your vehicle manager has had the option turned on when you don't require it. (or a jumper has been removed)

i don't have a wiring schematic but i have worked on these trucks before and you can get by without it, just follow the wiring with your meter to the vm plugs.