1988 Chevy K1500 1/2 ton 4WD

I have a 1988 chevy k1500 1/2 ton 4wd with a 350 I believe it is stuck in 4 high the reason I think this is when I drive it over 50-55 it overheats and it won't shift into overdrive I have tried the water pump, the thermostat, and checked the tranny fluid and none of this has seen to help the overheating problem or the shifting. How would I go about finding out if it is stuck in 4 high and how to get it out