To add to tclederman, before you take the shift box lid off, make sure that the rubber grommets on the shift arms which are attached to the shift box are sound. Deteriorated grommets can cause the transmission to shift into two gears at once which will lock it up. Not a big deal, just annoying and easy to fix with new grommets.
Crawl under the truck and manually put both transmission shift arms in the center position (that is neutral).
Now take a look at the shift arms at the shift box. If they are not both in the center position (neutral), adjust the shift rod ends until they are.
Hopefully the new grommets and shift rod adjustments will fix your problem. If not, get back to us and we'll walk you through what to look for inside of the shift box.
