Most 4X4 have a transfer case with a solid gear coupling front and back axles making the system part-time in 4 wheel drive. They are intended to operate only on slippery surfaces like snow, sand, dirt and grass where one or more wheels can slip a bit and release the drivetrain loading. They can travel in a straight line ok, but turning left or right on an asphalt surface will cause overloading of the front axle, transfer case and rear axle.
The turning radius is very different for the front and rear wheels but they are forced to drive at the same speed so the whole drivetrain is under extreme load on a no-give surface like asphalt. Initially the tire grip will be the weak link and release the loading with a noise. The drive gears and selector in the transfer case will wear quickly and when worn enough will be the weak link and be forced out of mesh and out of 4 wheel drive. Many transfer cases use a chain drive and the chain wears and stretches enough to ride over the gears making a lot of noise and not much go. Sounds like you have a gear drive though.
To drive in 4 wheel drive mode on asphalt like surfaces you need a full time system which has a differential in the transfer case. The full time systems usually have a differential lock to facilitate full drive to front and back axles under adverse conditions.
Hope this helps.
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Art
2000 Taurus SEL Wagon 3.0 Vulcan (daily driver)
1993 Taurus GL Wagon 3.8 Essex
1991 Mercury Grand Marquis LS 5.0
1986 Chevrolet 3/4 ton pickup 4x4 350 with turbo400 and 208 transfer case
1978 Chevrolet 3/4 ton pickup 4x4 custom 6.2 na diesel with turbo400 and 203 transfer case
Planning 2008 Taurus X in a year
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