Really bad idea.

As I recall, the first Dakar racers didn't work out when they modified X3s and X5s, so they moved to full race vehicles with custom frames that looked like X5s.

First thing you need to do is find a way to add a real frame, instead of the unitized construction. Then you need proper locking differentials. Additional ground clearance. Skid plates. The list goes on. Make sure you relocate all the electronic modules up away from water level, as one poster found when he waded his X5 across a creek and ended up having to replace some very expensive electronic parts.

If you do all of that, you end up with something that still won't be very good at off roading, but which in addition is no longer a functioning X5, with all that an X5 has going for it.

So, recognize that it is a car and not a truck, put reasonable 17" tires on it with a decent tread and good strong sidewalls, and stay on roads, off trails, and out of the water.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue