Well, on the fuel tank side of the trans, you should see a rubber plug. When you pull that rubber plug, you will see inside the trans. As you spin the spur (in the direction that moves the tires), will will see a notch on the gear inside the trans. When you see the notch, you hold the spur so it doesn't move, then lift one side of the truck and slowly rotate the tires (while the other side tires are on the ground). You will see the shifting mechanism go by when looking at the notch. You will see two allen screws (or dark holes) go by. If you miss them, don't worry, just continue to spin the tires, they will come back. If I recall, the first hole/screw holds the 2 speed on the shaft and it's a "straight in" hole. The second you will see will be at an angle and that's your adjustment screw. It requires a 2mm allen. You will turn it CCW to make the shift happen sooner and CW to make it shift later. Sounds like you need to back it off 1/4 turn or so. It doesn't take much.
I'll have to dig and see if I can find a two speed trans to show you... although, it's so confining and the holes are recessed, a flash/camera may not take a good pic.
EDIT:
Well, unfortunatly, the two speed trans I do have has the stupid two speed mechanism upside down, so the pics are 100% accurate for you. I must have installed it backwards the last time I messed with it... may explain the shifting problem I had...
Anway, you should be able to get the gist of it:
Transmission plug:
Plug opened. What you see may vary, but this is what you see if the fore mentioned notch isn't visible:
The notch:
The first screw that holds the two speed on the shaft, you want to leave this one alone:
The second screw which is the adjustment screw should angle down. In mine, it angles up because I put it together wrong:
Hope those help.
