I'm highly opposed to this bad, bad idea. All wheel drive cars are designed AS SYSTEMS, and as such will not function as the manufacturer intended when critical subsystems and/or components are removed or disabled.

At a minimum one would need to figure out how to disable the X-Drive software (if you have it) and/or the DSC software. Both are active technologies that will automatically intervene in direct wheel control when slippage or an understeer/oversteer situation exists. If the software cannot actively connect to the wheel because you've PHYSICALLY DISCONNECTED THE HARDWARE, then you're asking for all sorts of trouble. I guarantee you that disconnecting the front axle was NOT one of the test-case scenarios that the BMW engineers had considered during their development/testing phase and without X-Drive and DSC working for you whatever minor advantages you were hoping to gain by disconnecting the front axles would be FAR overshadowed by the dramatic reduction in handling (and safety).

Additionally, if you're hoping to put all of the torque to the rear wheels understand that you cannot even physically do this in an X5 prior to the introduction of X-Drive (2003/2004), as NON X-Drive systems can only send a maximum of 50% of the torque to the rear wheels due to the purely mechanical nature of the transfer case (whereas the X-Drive systems can send 100% to the rears due to it's more efficient hydro-electromechanical design).

Don't f*ck with it...