Thanks for the quick responses!
Overall, it sounds like I might be OK until I brake hard, but I really should replace the other two tires on the front axle so that I have four identical tires.
We have a short trip (200 miles each way on interstate) planned next week, so I am going to risk it and tow it with the two different load rating tires (and hence PSI) - to avoid the hassle of changing the tires and $275. Ben Franklin's words are echoing in my head: "Pennywise and pound foolish". At least the tires are both radial, from Carlisle, and have the same tread pattern.
Thanks everybody!