Welcome and congrats on the new truck!!!
As for "making your trailer to high", do you mean the tongue of the trailer? If so, does your weight distribution (wd) hitch have adjustable height? The hitch head (part ball mounts to) would be botled to the shank (part that slides into truck receiver). If so, just adjust the head down to level the trailer out.
It is possible you may need a different shank that allows the head to be dropped more. Available at your local hitch/ rv dealer or online from places like etrailer.com. A new shank can be $100-$150 depending the size "drop" you need. One thing to consider if you need a new shank, get a 2.5" shank so you don't need the adapter to fit a 2" shank.
As for the 20" rims, they are basically the same overall height as the 18", and probably the 17" as well. I know because I am seriously considering changing to the factory 18" rims for a better tire selection compared to what is available in the 20" all terrain type tires.