most tire company list the true tire size on their websites. so pic the particular tire you want, then go to the respective manufacturers website to look at what the true size is.
however tires like the current Definity Dakota's you have now are an off brand tire made by a third party and do not have a manufacturers site to look up info like this.
I.E. my 38.5x16-15 swampers are listed as actual size of 38.2. not much, but still slightly smaller than listed. this is unloaded weight however. by the time i put the weight of my rig on them at 8PSI, the end up at only just over 36" tall. i'd bet unloaded, your current tires are closer to being 30" or just over, but the weight of your jeep causes them to squat some.
1998 ZJ-- lowered a touch
2005 GMC 2500HD 496 C.I.
Fat Amy-1989 K5 blazer. Linked, lifted, 38.5's on won tons. Big Girls can play in the woods too.