Truck not shifting into 4-wheel low

I have an '03 4x4 with 96,000 miles. I had a guy interested in buying it and wanted to test drive it. I have only engaged 4 wheel high twice and 4 wheel low once- all off road. So we were in a parking lot he engaged 4 wheel high with no problem. Then when he shifted back into 2wd it seemed like it didnt wanna come out. So we backed up a few more feet and it finally disengaged. Then he put the truck in neutral and tried to put it in 4 wheel low. It made a grinding noise. Then I put the truck in park and it went in. I looked in a manual and it said that driving on pavement for a period of time could cause the driveline torque to not let the transfer case shift. Did all this happen because of the pavement?

Anyone know what this could be??