Yea, I've got a 2wd prerunner, and also guess that toyota doesn't think its necessary on anything except a auto 4wd. I know where its located but it doesn't light up when the key is turned on. I would think it doesn't have a bulb on the cluster for it.
Went through the manual and it states only 4wd models have the warning light.
There is mention in the Toyota Factory Service Manual that the "D" gear indicator on the cluster should light up when the system enters temp check mode, fluid comes up to proper temp, etc. Through performing the procedure however, myself and others have recognized that the "AT TEMP" indicator is the light that actually illuminates and assumed there was a typo in the Service manual.
In light of this development (no AT TEMP light on PreRunner), maybe it was indeed a typo for the A750F (4wd) procedure, but correct for the A750E (2wd) procedure? I assumed since the mentioned light was incorrect on the 4wd procedure, it was incorrect on the 2wd procedure as well.
Everything is still the same for the A750E (2wd) procedure except that instead of the AT TEMP light illuminating, maybe the "D" gear selector should illuminate? I guess we won't know until someone with a 2wd volunteers their time and passes the info on to us.
As for the draining of only 1/3 to 1/2 of the total fluid - automotive engineers take this into account when they schedule the ATF maintenance plan. I personally believe that the 100K mile service is too great and have already performed two drain and fills with 94K miles on the truck - and will likely do another in the near future.
If you be proactive with your preventative maintenance, it should be very unlikely that you would run into trouble. The service only costs me $20 for three quarts of fluid and less than 45 minutes of my time.