that's a very common problem on these old trucks - for starters, open up the shift box on the column where you jack around the linkage, clean and re-grease it and see that the spring in there isn't broken - make sure the connections of the rods to the levers on the box aren't sloppy, might need to replace the bushings there [and at the trans] -
check your shop manual for adjustment of the shifter up top
Bill
