I think, it's not that easy, not to mention the transfer case of the threadstarter is the NV125, which is a full mechanical unit, so there's no actuator at all.

The X5s transfercases are planetary gears, so it's like an open diff between the axles - putting the fron driveshaft off would mean no torque at all will be transfered to the rear axle at all, because the rear axle will always have 100% traction - so my understanding, which may be wrong.

The ATC500 has the same basic architecture exept for torque vectoring, so in case of electronical faliure, it would act as the older NV125 with 62/38 torque split - so the putting off the actuator wiring would turn the ATC500 in to NV125 - just in my suggestion, I'm not sure.

So for me the only way to turn the X5 into RWD seems to be the install of regular RWD gearbox and custom propshaft.

I even barely think, a RWD X5 would get really much better gas mileage, because the AWD on E53 is very light and easy.