I am the original owner of a 1996 XLT 4x4 extended cab and have had this problem since it was new. I now have 90,000 miles on the odometer and you are correct that the grinding happens about every 20k miles. The first time I had this problem the dealer replaced the hubs. The next time I did some research and found that all I needed to do was remove the hubs, disassemble and clean them. This worked and was not difficult at all. The most important part of this though is DO NOT LUBRICATE the clean hubs! If you grease them up you will still have problems.
