Yeah, that's an impact driver, not an impact wrench. Not quite the same thing. I mean, functionally they are similar, but 1600 lb-in is not very much for breaking loose rusted nuts. Can't say how much more you'd need, but a typical full size impact wrench is in the 400-1000 lb-ft range. And some of the smaller ones are still double the torque of your impact driver. If you already have an air compressor, and air tool would be pretty cheap. If not, unless you plan to do other work on your 4runner yourself, it's probably not worth investing in a real impact wrench just for these bolts, because even the cheap ones aren't "cheap", so your best bet would just be finding someone who has one.
I went through basically exactly this with my running boards. Couldn't get the bolts to budge with my 18 volt impact driver, and had to use my friend's air impact to get the bolts off. And after that I bought an impact wrench as soon as there was a sale I liked.