If you want to gain the best performance, install a cooling fan and it'll freeze your fluid! ha, joking. If your traveling from eastern WA, why not take 97 through Weed to I5 then down to SD? I'm sure you'll be taking out 2-300 miles out of your trip thats for sure.
Towing techniques, you should do a lot of engine braking and using your brakes a lot less because our 3rd gen brakes aren't the greatest wonders of the world lol. I'd also recommend you put more weight in the 4Runner VS putting more weight in the trailer. Last thing you want is the trailer pushing you around. Also, having more weight on the drive wheels gives you better control, especially on snow and ice since I'm guessing you'll have a fair bit of that.
If you have a lil more weight towards the rear end of your trailer past the axles, you'll experience trailer sway, and let me tell you, its not something you want to experience. It can yank you sideways and push you into a jack knife. Don't starve the engine! Don't floor the gas peddle if the thing is hunting for gears. Down shift manually and keep it in a lower gear untill you crest a hill and engine braking comes from lower gears.
When I tow in my 4Runner, I open up the intake box to let it breathe easier. I've towed a car trailer with a 3500lb Cressida in it from Florida in my 4R in O/D accross the flat highways and with power mode, the truck down shifted as soon as it sensed there was going to be excess load from the slight uphills.