I hurt my back about 3 years ago also. One of the things I hate about the 4Runner is the seats sitting on the floor. I have NEVER been able to get comfy in these seats. I even wrote Toyota about this crappy design. The seats kill your back if you are taking a 300+ mile trip.
One thing that has help me when traveling is to get one of those long (approx. 24") travel pillows and place it under my knee/lower thigh area. This lifts up your legs as if you were sitting in a truck bench type seat. This is and may be the MAJOR reason I upgrade to the newly redesigned Tundra truck or the new designed 08 or 09 Sequoia. They MUST have seats that do NOT sit on the floor like the 4Runner and Tacoma. If not..... I'd have to consider moving back to an American product.