My wifes 97 XLT started a making a popping/grinding noise when turning to the left more than 45 degrees and the 4 High and Low dash lights started blinking 6 times, ocassionally, when driving. We took it to the Ford dealer and after an $85 diagnosis, we were told it was a loose wire near the transfer case causing the front drivers side hub to try to engage, and were charged another $45 to fix that. I was then given the car back and sent on my way. The car had no problems for one full day, then the same situation returned, when my wife started driving home from work, after calling me, we decided she should try driving it in 4 High, which seemed to help as it was raining that night anyways. My wife returned to the dealer and was told that we now need to replace the following items: Wiring Harness Jumper $60, Front Speed Sensor $64 Rear Speed Sensor $78, Shift Motor $267 with 3 hours of labor totaling $261 for a Total of $730 and tax.
My concern is now that they seem to be guessing as they told us to try replacing the harness and speed sensors first and see if that fixes the problem before replacing the shift motor. I have that bad feeling that we could spend another $730 and the they would come up with some other possibilities if these don't work. I already feel like they owe us atleast a partial refund for being wrong on the first diagnosis.
We have only had this Explorer afew months and have already replaced the upper and lower ball joints, control arms, and tires. We like the car and are planning to keep it but this is a big chunk to spend on an older car when have already spent quite a bit, especially if we don't have to. I would appreciate any advice anyone could give me.

Thanks,
Randy