won't go into Drive?

[purple gas]

Like it says. I can shift the lever into D, but the tranny won't shift. T-350, it is. Should I swap it out for another one? Could it be a simple thing? Should I hit it with a hammer? Are any carb linkage or vacume lines involved? Could it be a pull the pan and check / change something simple? Does it have anything to do with the fact for 1 revolution of a tire, the drive shaft rotates not quite exactly 2 times? Any advice would be greatly appreciated. FINALLY bombing around in my truck is fun, but avoiding the freeways sucks!