Factory rims are 15" in diameter and either 8" or 7" wide with 4" of backspacing and a 5 on 5" bolt pattern. 5 bolts, 5" circle.
235/70/15 tires are generally 28" tall and about 9.25" wide (not tread).
The size of the rim diameter is irrelevent as long as it isn't bigger than 28" in this scenario. What does matter is the back spacing and the width, of which the 18x10" rims are going to be 10" wider. If it has less than 4" of backspacing, and it is lowered you are going to have issues, more than 5.5" and you will have issues, so there is a limit on the backspacing. In addition, depending on how low the truck is and how big a tire you plan to run you will probably have rubbing issues on the inner fender.
With all this said, you really need to measure off your current setup and see what backspacing you should go with. On the height, if you decided to stay with the same current height of 28", you could put a 255/55/18 and the width would be. 75" wider and 1" taller, or you could put 255/50/18 and be. 75" wider and right at 28" tall.