Terry, I couldn't find anything like that either. I "upgraded" my Jamar setup (at least in my own mind), by installing a heim joint through the floor that supports the rod right aft of the shift box. I installed better U-joints in the system. I use a cotter pin at the first u-join in the engine compartment so that I can take the front half out if I ever need to. I have a heim joint support at the trans pass through and I cut the last rod and installed a coupler in the back section before it gets the nose cone articulation - this is so I can do further adjustments such as lengthening the rods without fighting the front and back against eachother. It is my contention that the slop in the system can be reduced greatly by supporting the rod system in a couple of places and by replacing the u-joints for tighter ones. The toal cost to do this is mostly time (maybe $25 in parts).
I do exactly as you suggest.
First, make sure your tailcone articulation is tightly clamping the tailcone, that the part attached to the rod coming out of the trans is tight (it has two set screws) and that all joints, pass- throughs and the shiftbox are greased. I disconnect my system front to back at the coupler. If you can't disconnect the system like this, then just loosen the front adjusting nut on the rod just aft of the shift box and do the same. Don't loosen front AND back nuts or you are fighting your self. I use the front nut for rotation adjustments (left/right). and the back locknut for length adjustments (fore/aft) but hey that's me. I put it in a gear (I like reverse because it is at the extreme of the pattern) by manipulating the tailcone articulation. I put the shifter where I want it to be for that gear and then find a way to hold it there (bungie cords, small child, drift wood, dead animal, etc.). Then I snug everything up and check it. If it isn't right, I loosen the nut and try again.
If you are having troubles getting reverse and second then most likely your rods need to be shortened by adjusting out at the rear locknut near the tailcone articulation. If you are having problems with 1st and 3rd, then lengthen it a little. If you can't get reverse, but all others are okay, it could be that your side-to-side adjustment is off.
Keep in mind that inside the shiftbox the action is rather minute. Take a look at how little the rods in back rotate to go from the 1/2 gear plane to the 3/4 gear plane. It doesnt take much slop to ruin this. The sweet spot for dialing this in a rather small and might take a little while to get, but properly done the action is nice and precise.
Does this help at all?
Mike