crawl under your truck with a set of sockets... disasemble all the linkages from the shifter in the cab to the t-case... clean them... I usually use a wire brush that is mounted to my table grinder to remove all the tarnish and rust. Then lube all contact points with WD40, white lithium, tranny fluid... LOL.. anything that lubes... I usually use bering/axle grease b/c it sticks a long time and won't dry out... Reassemble all parts... 85% cure rate for not going into Low Range...

You can use a spray lube to get you down the road but eventually it will dry out and the rust will have you all gummed up again... take it all apart and you'll last a lot longer. Plus you'll get to inspect the pieces as they come off...