Hello all,

Long story short I'm in the market for a vehicle and have found a really nice low mileage '01 explorer sport. ~118000km on the odo
It is a single owner vehicle that has been well maintained.

The owner claims that he rarely, if ever, used the 4x4 (we live in sunny southern British Columbia where the road conditions are rarely anything worse than wet).

During the test drive I activated the 4hi at ~50 km/h on a wet road (to ensure it worked properly). What followed was a dull bang/thud and a considerable lack of power. After stopping I disengaged the 4x4 and drove on a few yards and the noises/problems persisted. At this point the owner and I decided to proceeded to my mechanic (we we're just leaving from his shop when this happened).

My mechanic discovered that the vehicle could be put back into 2wd and would run perfectly fine after backing up several feet. He also found that whenever the vehicle was put into 4wd (high or low) the problem would surface again.
He thinks that by not using the 4wd a problem could have developed in the transfer case.
While driving with the 4wd engaged there was a considerable wobble/shimmy that worsened with speed.
When my mechanic was driving past us trying to diagnose the problem we noticed that the one of the rear tires would intermittently lock up and drag. (I think this could account for the wobble)

The vehicle owner had replaced a pair of tires on the truck with the exact same make/model. There is a bit greater than 3/4 life on the new pair and about half tread on the older pair.

We all seem to think that the difference in tread depth and the lack of use of the 4wd have lead to this problem. We also think that these two things may have caused a deeper issue in the transfer case or the hub.

Has anyone else experienced this issue? What was the problem and fix?

Im still planning on buying the vehicle contingent upon its repair...