I had Falken Wildpeaks on our GL450 with 18" rims and that thing went anywhere I wanted it to! Great ride on the road as well, which is where it spent 90% of its time. I was able to get slightly oversized to fit. IIRC I think on the X5 the upper control arm limits the size of the tire? Consider going down to 18" wheels, which are lighter and give you a lot more sidewall which lets you air down more without damaging like a 20" would.
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