ok, take the body off, look at the left side of the truck (drivers side if you will) looking at the side of the tranny, there will be a rubber plug that covers 2 holes https://www.rcnitrotalk.com/gallery/showphoto.php?photo=3329&size=big&papass=&sort=1&thecat=
that pic shows what I'm reffering to, ok pull the plug, you may need a flash light to look down in the hole,you will be looking in the bigger of the 2 holes, the one toward the rear of the truck, set the truck on it's right side (passenger side), with your finger rotate the big spur gear until you see the hole come around, this is where the allen wrench will go to get to the set screw, now with one hand hold the spur gear so it doesn't turn, with the other hand rotate the left rear tire in the forward direction while you look down in the hole ( you may need someone to hold the light for you) rotate the tire and soon you will see 1 of 2 screws, don't touch the 1st screw, rotate to the 2nd, this is the 2 speed shift adjustment screw.......if you want it to shift sooner, turn the screw 1/2 turn counter clockwise, for a later shift turn 1/2 turn clockwise, adjustments should be made in 1/2 turns.
Havnt had time yet...working 14-16 hours a day and dont expect a day off until Xmas,then not until the 1st...if it rains on either day I may just cry...:>)