Look underneath your truck, up by the transmission. You will see the linkage rod going from the shift lever, to the arm on the transmission. Somewhere on the linkage, you will see an adjustment. On my machine, there is a bolt that 'locks' the adjustment where 2 linkage pieces join together. Loosen the bolt, and you can slide the linkage pieces against each other (to make the link longer or shorter). You will have to use trial and error to find out exactly how much you need to move it and which way.
JB