On the 4 speed transmission the third gear is 1:1 and on the 5 speed, the fourth gerar is 1:1. The last gear is the overdrive and yes, the overdrive works for towing lighter weights, but even in level ground it tends to build up heat. That is why some people who tow get a transmission cooler, the transmission builds up heat when you tow. My feeling is to use the overdrive when it is flat or downhill and you are going pass 60 MPH and even then, turn it off for awhile, just to cool the transmission down. My parents had a GMC truck and the transmission went out when towing. The dealer said, they should of towed with the overdrive off.
The transmission is built pretty tough in a 4Runner, but be mindfull of what it is doing.