Towing
I am not 100% sure but not more than 3000. The car is solid steel of cousre and the darn thing is a bear to try and push/load.
I beg to differ. I'll bet you a round of Beefeater Martinis that you're at or above 5,000#. That cast-iron U-Haul is at least 2,000# and I'd be very surprised if the car wasn't another 3,000# at least. The '03 Runners are only rated to tow 5,000#, of course.
I tow a Porsche 911 track car on a Trailex aluminum open trailer, and even that lightweight combo weighs in at 4,000#. I just got back from a trip from Michigan to VIR near Danville, VA, which involved lots of miles in the Blue Ridge mountains. My '03 did a wonderful job, including descending a long 9% grade with no brake problems, which used to completely fade the brakes on my '98 prior 'Runner. But, then, I used the gears to help brake the rig.
I would be VERY sure the trailer brakes on your rental are in working order, and be prepared to manually downshift to a gear lower than you think you'll need when descending steep grades to save the truck brakes. I'd also be sure that the shank of the ball is properly sized for the big hole in the ball mount (drawbar), and that the nut is torqued to the appropriate figure (250 lb-ft for the 1" ball shank, 450 lb-ft for the 1-1/4" shank).
I think you're going to be pushing the limits with that tow, and you'll want all the help you can get. Good luck.
-Karl