This has been covered at length before in this forum, and I've had the problem on my '55 1st 1/2 ton. That shifter box is a real bugaboo. Pull the cover, clean out the old grease, and adjust the shifter arms so that the openings match up when you are in neutral. The linkage adjustments are very critical to prevent the lockup you describe. You go all through the shifting system, adjusting all the way up to the hand shifter in the cab. This is well covered in the shop manual, which you can reference elsewhere in the forum...pics and all. When you're done, lubricate the heck out of everything and pack the box with grease.
