I'm not surprised that there are a lot of responses to your post. I may echo another's comments. Forgive me; I didn't read all of them.
You mention the tire/wheel packages being 20's instead of 18's. Look closely at the package...not just the wheels. It's likely (not guaranteed) that the tire circumferences on the 18's and 20's are nearly the same. The 18's are likely to be fitted with a higher profile tire....say a 70 or 78 instead of a 60. The 20's are likely to be fitted with 60 or 65 profile tires. The result is that the tire/wheel packages are essentially the same diameter/circumference from one to the other. That means the truck will be about the same height with either tire/wheel package.
That said, the 18 will be a slightly better "truck" tire, because the taller profile means it has more distance between the rim and the tread to absorb sharp-edge impacts. This difference is not substantial, but it can help in the really rough stuff.
OTOH, I have 20's on my RAM 1500 that are fitted with 65 profile tires, and they are more that beefy enough to absorb climbing curbs, rolling over sharp rocks, and so on. These are not "rubber bands" by any stretch (pun intended). All I do is boondock, often in some pretty harsh terrain, and I've never had a tire failure due to an impact. (Carpenters, on the other hand, need to learn to clean the loose deck screws out of their truck beds before they go camping!!
)
Anyway, that's my 2 cents. The truck is very likely to sit at about the same height with 18's or with 20's. Double-check to be sure.