Look at the concern in a logical manner..... after driving over an object,
you now have a concern. The only advice I can offer at this time is that you
visually inspect for any physical damage. While there are "pattern" failures
for many of these systems, we have what may be a cause and effect scenario
in your case. I suggest a thorough inspection of the driveline and transfer
case as well as looking for any obvious signs of wiring harness damage.

Well, if you can't get any technical support in France you could still
probably buy a Hayne's or Chilton's manual.  Sometimes they can be found on
eBay.

From what you described my first thought would be that perhaps the wiring
for the servo motor for the transfer case is damaged (or possibly, but
unlikely, the motor itself).  The servo motor is located just front of the
middle of the truck, underneath.  Aside from that the servo motors have a
great tendency to fail and maybe it was just a coincidence.  I've had both
servo motors fail (the gears inside get stuck) on my '91 and '92.  Usually
when they don't work (on mine)they will go from 4x4 to 2x2 but won't go back
to 4x4 unless I fiddle around with it.