I had this problem once. 1st gen explorers use a cave-man style lever and cable assembly for the ac and heater controls. Later they switched to an electronic control system. (ours is simpler to work on, hehehe). In some cases these become stuck inside--you can still move the lever normally, but inside it's not catching like its supposed to. In my case it was not the switch, it was the plastic piece inside the dash that the a/c and heat control lever was attached to. It's a plastic piece with notches in it. If the lever slips on the notches, the a/c will not disengage.
When I moved it to defrost, which automatically turns on the a/c, the a/c wouldn't turn off again. I replaced the switch under the hood, thinking it was that. But that did not stop it. Only after I dug into the dash, and lubed the plastic piece with silicone, did it turn off. It was a pain getting in there just to lube it, but it's worked ever since.
Let me distinguish here: there are 2 switches, one under the hood (which is pressure operated on the a/c line) and one in the dash. In my case, the small switch in the dash that the lever is attached to was fine, but the part actuating the switch itself was bad. The underhood switch was also ok.
