Really? I thought all Trucks of this size range where fully buxed for like 20 years? Was this the same for the Titan? Because all the 3 big US companys where fully boxed so much longer?
Toyota used fully boxed frames on their U.S. truck until 1995, which is when we got the Tacoma, which diverged from the global Hilux. Some parts are shared and the frames are similar in front. But the two are distinct designs.
Tundra forked from the T100 that was a scaled up Hilux/Pickup. The Tacoma has always been open C from the cab back. The reasons given generally boil down to a more "compliant" ride, meaning softer for Americans who use their trucks without much cargo.
The 95-04 Taco frame can do with some help if you ask it to carry weight a lot. These are the reinforcement plates I put on the one I owned for a while. The 2005+ frame is double wall 'C' through the middle so there's less need to do this. It's still not a particularly rigid frame but it's strong enough.
The camper I have on mine was about 10 years old when I got and put it on the 1991 for 15 years, 1 year on the reinforced 2001 and now 7 years on the 2008. The fatigue and fracturing in the fiberglass has noticeably increased in the past 7.
Now it's 33 years old and the first 26 years surely accumulated plenty of wear-and-tear so the extra flex and vibration in my current 2008 can't be completely blamed. But when I put the rear on jackstands to work on the rear springs (so stands right in front of the forward fixed leaf spring hanger) I have to put a 3rd tall stand under the trailer hitch to keep the tail from sagging excessively. You can *hear* the fiberglass straining if I don't.
It should be mentioned that domestic trucks weren't always boxed. I know the square body GMs had C channel frames. And medium and heavy duty trucks have C channels. It's not a simple yes-or-no as far as strength. I do think boxed frames are generally are more rigid, though.
