I too have a 1998 Ford Ranger that the 4 wheel drive does not engage. I have been doing some research online and have found one main reason for this. Since our Fords are shift on the fly 4 wheel drive the hubs automatically engage for us when we switch that little nob. Well after 100,000 miles of doing this the automatic hubs wear down and the 4 wheel drive is unable to engage. I have done some research and those automatic hubs are about $200 each. But you can get the manual locking ones for about $75 each. If you get the $75 ones you will have to manually lock your hubs when using 4 wheel drive, but they will not wear out like the automatic ones do and they are cheaper.