1998 Ford Explorer

Drive Train Axles Bearings problem
1998 Ford Explorer 6 cyl Four Wheel Drive Automatic 117000 miles

Not long ago I began noticing a chattering or slipping sensation in the drivetrain when acelerating. This problem seemed to exit only in Auto 4wl drive. At first the dealer told me it was the new tires that were installed at Walmart as there circumferences were not the same. I had one tire replaced but that did not solve the issue. I returned the vehicle to the dealer, now they are telling me that I need a new transfer case and driveshaft at the cost of $2,400.
I asked them if there was any way to disconnect the Auto feature and they did so by disconnecting a plug above the transfer case. Now the vehicle operates smothly in two wheel drive, but of course I cannot shift into 4wl High/Low.

I would just like to be able to understand this problem. I have it in my head that this problem is an electrical or sensor issue, not the entire transfer case. By the way this is a two owner SUV that has been serviced as recommended by Ford, including the transfer case. Please let me know what you think?
May 28, 2010.

Was the the problem only in A4WD? Did you put a different size tire on the vehicle?

The problem is onlly in A4whl. I have replaced the tires recently with all new Goodyear Wrangler. The Ford dealer thought this was due to differences in tire circumference, but now they say not. How can I check this? Did I say that they Ford dealer disconnected the transfercase motor, the drive is now in 2whl and is working without vibration? What do you think, they also want to replace the front driveshaft?

Jun 2, 2010.
Disconnecting the transfer case will eliminate the A4wd, but the vehicle runs in 2WD regardless until the rear wheels spin. Something tells me there is a output speed sensor that is going bad or is bad. If it thinks the rear tires are slipping, it will automatically engage the front axle.

Does the Automatic 4whl only evaluate slippage from the rear? If so, I know that the dealer is in the dark because the tire that I changed, as they advised, was on the right front. So how do I test the sensor?

Jun 3, 2010.