If this is for an AD truck you should be able to remove the adjuster pieces from the shift assembly (on the steering column inside the engine compartment). Little cotter pins and small washers hold them in place. Once you remove those, then get under the truck and put the transmission in neutral (by moving the shift levers on the side of the transmission). Neutral will be when each of those levers is in the center position (there are 3 positions for each shift lever...forward, center, back). Then when you have the transmission in neutral it is easy to turn the adjuster pieces (they are threaded) until both pieces of the shift assembly are at the same place (this will be neutral) and replace them. Hope this is helpful. NOTE: normally these do not get out of adjustment unless (a) the shift assembly was replaced with another or (b) one shift rod has become bent for some reason.
And to add to Jon G's very good instructions.....on the shifter box that it is on the steering column.....make sure your two levers are parallel with each other, BEFORE you install the shift rods at the top to the two levers coming out of the shift box. These two levers need to stay parallel....this is critical. And they are usually pretty close to horizontal when they are correct also, (which would be pointing towards the engine). You can verify this by taking the grease cap off the shifter box, and when the two levers are parallel, the two shift gates inside the box should be lined up with the shift tab (the tab is what moves up and down inside the box). There is not a lot of gray area on this either....it has to be pretty close to right for it to shift smoothly. You can be off a little and it will shift, but it will hang up more often than not.
On my 51 3100, the adjusters at the top of the shift rods just slide up and down and you tighten a nut when they are at the right position, more or less clamps itself to the shift rod. But on my 48 truck, these adjust just like Jon G says, they are threaded, so you have to screw/unscrew them to get the proper adjustment.
One other thing, you don't need any slack at all, on the inside of the shift box. Take the top cover off, there's 4 screws that hold it on, and you can watch how it works. Very simple design. There is a spring inside, rides on the shaft above the top shift gate, that keeps tension on all of it....not a stout spring but enough to keep everything snug.
Begin by manually setting both levers on the transmission in the middle (neutral) position. Picture #1 When both transmission arms are set to middle/neutral remove the top of the shifter box attached to steering column under the hood and ensure that the shifter forks are correctly aligned as in picture #2. The column shifter inside the truck should now also be in the neutral position. If column shifter is not in correct/neutral position, - loosen and adjust the rods to the transmission until all components (transmission/ shifter box/ shifter inside cab) are in neutral. Retighten all adjustment bolts. Shifter should work properly after this adjustment unless components inside fork box are worn and need replacement.
Not to insult your intelligence, but do not take off the cap to align the forks. Simple pop off the little round cap (grease filling hole) and observe/adjust from there. They should still look like the cap off picture in the post above.
ndkid275.....no offense taken. But just looking through the grease cap hole ain't gonna show anything other than whether the gates and the tab are lining up. Taking the cover off will show you if the spring is correct and if the gates have any slack in them. I have seen them needing to be shimmed.
One other thing to consider, the holes on the levers where the shift rods connect, these need to fit snug. I have seem rubber grommets used, and somebody may make a specific bushing for these, that I don't know. But I made some out of copper tubing and they work perfect.