My xxxnt has never really wheelied either. The front end comes up when i hit a bump, but thats about the extent of it. I get to much wheel spin. On pavement, it spins, on grass, it spins, on dirt, it spins... i think you get the picture. With all this spinning, i'm obviously not getting all the power to the ground. But i don't really care. I wheelie with my .21 maxx and drive circles around my friend with my xxxnt when he's driving his .21 maxx.
From my own experience, the losi trans is relatively weak. I think the few times i did get the power down, it cost me a trans rebuild. I've since put in the RRP Monstor diff gear and havn't had any problems. But i don't drive it as often due to the maxx. It's more funner.