Could be one or both of two problems. 1) Bent shift fork. You'll have to tear down the trans to fix this one. 2) worn doohickey on the end of the shifter. Pull the shifter and get it welded up and reground to the original shape. This is the easiest one to fix and the one I would start with.
You'll need some help getting the shape correct like a parts manual or one of the old timers that know what the shape should be, but it's still the easiest fix.