Since you posted in the HD section, I'm assuming you have an HD truck. If that's the case, your truck doesn't have a factory rear block. The only way to lower the rear is with lowering shackles.
Some guys are running 37x12.50x20's on stock rims with a 3/2 style kit and minimal trimming. What size of 38's are you planning to run, specifically? Wide (13" plus) tires on stock rims can be tricky. Without wheel spacers, you'll run into clearance issues on the UCA and frame (when turning). Adding spacers will fix those problems, but put the tires closer to the fender liners and mud flaps (and maybe the bumper) when turning. I don't have as much experience with the new style HD's, but that's how it worked with the previous generation.
I first switched to a set of LED's from www.xenonpro.com, but I wasn't impressed. The light color was whiter, but not that much brighter down the road. When one of them burned out, I replaced them with HID lows from www.diodedynamics.com. I'm very happy with them. They're super bright and pair well with my LED highs and fogs. The projector housings are much better for HID's.
I know it's been a while since you asked, but I'll respond just in case it helps. You'll have no problem swapping from factory 18's to factory 20's. Your speedometer will probably still be accurate, because the overall tire diameter is what matters. The rim diameter is irrelevant. You'll need to pair the new TPMS sensors to your truck, and you're good to go.
Empty, I'm getting 12-13 mpg in mixed driving. Towing, it drops to 9-10 mpg. The 2500HD gassers are much better at doing "truck stuff" than the 1500's, but you pay for it at the pump. Great trucks aside from the gas mileage.
They were very slightly used. There's a local place that deals in new and used takeoffs. Their previous owner traded them in on a set of factory 20's. Here's a link to their ebay store: https://www.ebay.com/str/joeswheelsilliopolisil
I was very happy with my last set of Falken Wildpeak AT3W's in the winter. I would definitely recommend them. And a 275/65/18 would fit without any trouble.
You're welcome. Some members are also running 35x12.50x20 or 295/65/20 on stock rims with a leveling kit, but that seems to be the limit. Those sizes will also rub at stock height, for sure.
If you're stock height, you can run a 275/65/20, 285/60/20 or 295/55/20. If you're leveled or willing to trim a little, you can run a 285/65/20, 295/60/20, or 305/55/20. Those are all based on the presumption that you're running stock rims. With aftermarket rims, that changes things.