97 Z71 sticks in low gear mode

I don't really know what to call this problem and I have asked everyone what the problem is. I hope somebody can help me. I have a 97 Z71 with the 5.7 350. When I have the Air Conditioning on and driving everything is fine but if I stop anywhere and the truck is ideling with the air on then something happens. The truck goes into some sort of mode that seems like it is really low geared. It doesn't shift out of first gear until the RPMs are about 4 to 5 thousand. It is really loud and the engine is really rapping out the RPMs. It continues to do this through the next gears. This problem can also happen if I forget to turn off the AC before I shut off the truck. When I start it the next time it may go into this mode or it may not. The only way I can get it back to normal is to go someplace where I can start from a full stop and put the accelerator all the way to the floor and go. The RPM go just about as far as they can go before it shifts and it sounds like the engine is going to blow and any minute. That is just for the first gear...after it shifts out of that gear I have to keep it floored until it shifts out of the next gear....which I am usually around 80 mph or above now. After it shifts hard into the last gear then everything is usually back to normal until I the next time I forget to turn it off or just sit with the air on.
Can anybody help me. Is this some sort of sensor or is it a computer problem. I am assuming, but by no means know, that the AC is putting such a load on the engine that some sensor or computer is telling the truck it is pulling something....but once again I have no idea what I am talking about.
Please...help me.
Keith

i dunno about a sensor and what not but i have the exact same truck and mine always shifts at 5-6K