My 2002 F150 4x4 seems to have developed a whine in the front end drive system. It always had this whine in 4 hi and lo, but now I am hearing it in 2 hi. It's like the system is not disengaging. but I know it is. The whine used to go away when I switched back to 2 hi and drove it for a bit, but now it stays there. Any suggestions ???