Are you sure about the 255/55/18 size? I don't think Nitto's come in that size. Besides which, the Terra Grapplers are A/T's. Are you refering to the Mud Grapplers? Those are really cool tires!
I have the Terra Grapplers and they are great. They are 285/60/18. About 1.5" taller than the factory 255/55's, and quite a bit wider. They are quiet on the road, great in the rain, good in the sand, excellent in rocks, and terrific in the dirt.
Other opptions include the Cooper STT's, which are really nice mudders, or the ST's which are comperable to the Terra Grapplers. I have also seen The General's "Grabbers" in 18" size, and they look like great tires also comparable to the Terra Grapplers.
In the US market, Goodyear says their well regarded 18" MTR's (sort of a cross between an AT and an MT) will be available soon. Of course, they initally promised September, so....
Here is a photo of the Goodyear MTR's
http://nwoods.smugmug.com/gallery/1968224/1/100140248 This is on a buddies LR3 from Vancouver, eh?