ive got 34x12.50x16 on factory rims, i have 1/4" spacer and i barely rub the lower control arm.
the bad thing with the unit bearings is that when you run a thicker spacer it will wear the hubs down faster.
this is all with a 2" level kit up front though, made a big difference. I am going to run 35"s next and get them 35x10.5"x16 this should keep rubbing to a minimum.
2001 2500 cummins long bed ex cab.