Pre runners are trucks that race teams use to pre run long desert courses like Baja. Based normally on stock trucks and built with long travel suspensions. Not as tough as full on race trucks but will handle much higher speeds and abuse than stockers. Think Ford Rapter. Normally 2wd. Much cheaper to beat up than the full on race truck. Some are getting pretty close to race truck capable though. Many people are building recreational trucks based on the concept and look.
In Toyota's case they are 2wd trucks that have a 4wd stance and some TRD upgrade pieces.