I have the same year and model 4wheeler but not a 4x4. I also have that same problem and cannot use high range right now. I have a bungee holding it in low so it wont pop out of it. I was wondering if you ever solved the problem.

 

I'd look in your service manuel and find the parts diagram for the gearbox/ transmission and look to see what parts are engaged in super low 4x4 and 2x4 range. Then I'd look to see still using the parts diagram see what the shifter forks would engage as you move the hand shifter through the positions. (Low, super low, foward, reverse, neutral )
I'd try to figure out what gears and shafts are engaged in super low and see if any of those gears cause any of the other parts in the transmission ranges that dont work to rotate. Figure out how the transmission works in super low then try to figure out what engages to what to let the transmission shift through the gears.. that should tell you your problem. Could be a problem with the shifter forks or the clutch cable could be improperly adjusted. Be sure to check to make sure the clutch cable is properly adjusted first. Your service Manuel should tell you how to check it. Also I'd read the entire section on the transmission too.

 

Everything seems to be engaging properly in high and low, no super low on this model I believe. Adjusted the clutch today and is shifting much better but does not appear to have done anything for the problem. IT is however staying in low without any issues. Will take off in high but then popps out within the first few feet of driving. Have to rock wheeler back and forth and put back in either high/low to go again. High consistently popps out. Im wondering if the cable is stretched. Ive read some that say you can adjust the cable itself but looking today did not see any place to adjust, am I missing it? Considering disconnecting cable and attempting engaging high by hand and then seeing if it will stay. Will this hurt anything? If this fixes the problem than a new cable is in order, if not I may have a deeper issue... Open to suggestions. All help is appreciated as I am new to ATV's and have limited mechanical ability.