The HG is a major job(about $1500-2k at a shop) on the 3.0, so you will want to make sure you have taken care of all of the little things first. It sounds like a fuel air mixture problem, since it is o.k. at light and full throttle. Look for a leak in the intake after the air cleaner or around the throttle body.

A bad HG would/should be a loss of power throughout the entire power band, though a compression check should be done to verify no HG problem. A valve shim(adjustment) could help, and is about due at over 86k, but I don't think it would cause the problem you are having, a stuck valve should also be a loss of power throughout the power band.

Check the engine codes, if a sensor is going bad it may have registered a code. The FSM(factory service manual) at the top of this forum should help you out.

Do you have a 5-spd or auto tranny? What gears do you have?
The 32's will have an effect of a loss of power, how long have you had the tires? Gear size will also effect the performance, more if you went from 225's w/4.10 gears for 5spd, 4.30 for auto(stock) to the 32's, and not as much as if you went from 31's w/4.56 gears for 5spd or 4.87's for an auto(oem upgrade) to 32's.