This is not a good idea.
First of all, if you are used to the 255/70/16's, then be VERY careful driving your truck, until you get used to how it feels with the 235/70/15's. The "new" tires are 20mm narrower (which is a huge change), and will substantially reduce vehicle handling and stability. It will be even worse if you have a 2dr Sport, asd there short wheel base already makes them a bit "squirrelly" , particularly at highway speeds and in agressive turning situations.
Further, the 235/70/15's are about 8% smaller, in rolling circumference, compared to the 255/70/16's. Your speedo will be off by 8%. Since the "new" tires will be "shorter", your speedo will read "high". In other words, if it SAYS your doing 60mph, you're really going 55mph (92% of 60mph). Given the complex relationship of the various vehicle systems, this could be enough of a difference to cause a problem. Trans shift points are determined by the computer using various factors, including vehicle speed. If it has incorrect speed info, it could cause a premature upshift or late downshift, lugging the engine and reducing performance, and possibly causing long term damage to the trans. (due to variations in intended line pressure/clutch apply rates,etc...) This erroneous speed data could also cause issues with your ABS and 4x4 systems, since they are also speed-based systems.
