It sounds like you've covered your bases fairly well - is the fan shroud intact and installed correctly? If it is, then the problem is just too much load and not enough speed to carry the heat out of the radiator.

Just to be clear, is your coolant mixture 70% water/30% coolant or 5-1 ratio as mentioned in the other post? 5-1 (roughly 85%/15%) will dissipate more heat, but either way, the more water in the mixture the better your radiator will transfer heat.

Also, keep in mind that it's normal for an vehicle to run hotter when it's under load like that. It's within a safe operating range unless it's approaching the danger zone on the gauge. Even 3/4 of the way up the gauge would be normal on a slow uphill climb.