On the YJs, there are two places 4wd gets engaged and disengaged. The first is the transfer case, and that is what is most directly activated when you move the shift lever. Secondarily, there is the axle disconnect in the front axle, which is what this thread is about. To get the best highway performance, you want them both disconnected. What this does for you is it lets the front drive shaft and half of the differential stay still while you're going 70 mph or whatever. The front drive shaft is the thing that connects the transfer case to the front differential. If you have the permlock, then when you shift into 2wd the transfer case is disengaged, but the front drive shaft still keeps spinning. This wears your front tires out really really fast. But at least, you're not going around with the massive tension in your drive train that you would have if you had 4wd engaged on the highway.