need some help guys!

i got a 2003 Z71 chevy tahoe. the damn thing wont turn to the left when i'm stopped or going slow.. it turns fine when i'm going down the road. turning it the the right is no problem at all.. i check the power stearing fluid it was full.. weird thin is when i put it on jack stands it turns fine both ways. so it only when its on the ground turning left going under 5 mph... ANY help would be greatly appreciated......  

it sounds like the steering wheel position sensor located at the bottom of the steering column in engine compartment, up next to the firewall. the sensor makes it feel like there is something binding up turn either way. it usually happens at very low speeds turning or going over bumps. its lokks like you have to get at the chevy dealer and i believe they are about 65.00$