Up here in snow country, I've replaced the front calipers at least 3 times and the rear calipers at least twice.
Living in NH....I just recently replaced the rear calipers for the first time on my 05 4runner with 240k miles. The fronts I've replaced several times.
I'm amazed that you're experiencing problems with the rear calipers because they are a completely different setup then the front. The single piston rear caliper should easily last over 200k miles. No 4runner owner I know of has had problems with the rear caliper.