I have standard 275/65/18 on my truck now. I would like to go up to a 20 inch wheel. What size tires do I need to get without increasing the overall diameter of the tire?

 

275/55/20 is the OEM offering for both 2wd and 4wd trucks.
The slightly taller 275/60/20 will fit stock trucks also. It's roughly 1" taller than stock, won't throw off the speedo/tach/odometer much at all, and it's readily available in a P-metric load rating to retain a factory like ride.

 

Good tool...
https://tiresize.com/chart/