Ed & Harvey,
I too had 2 Uniroyals throw treads in the middle of Mexico and ended up driving 300 miles to Laredo on a pair of VW Safari tires, the only 15" diameter available in Mexico at the time.
I hope there is a tire expert out there who can tell us for sure about rotation of radials.
The only tire rule I can confirm firsthand is NEVER mix radials and cross plies. Even on separate axles!