Aaronb is right, but I can only speak for the smog laws in California.
You must keep the same chassis designations. An engine from a 2WD truck can not be legally transfered into a 4WD truck chassis.
Please take this story as just 1 experience I had. This is a word-of-mouth story, but hopefully it will give you a better idea of the two.
I do not have any personal experience with the 2RZ. A customer of ours did a 2RZ conversion shortly after I had done my 3RZ conversion, and he was asking me what I can pull the grade up Hwy168 at, and with similar setups, 35" tires + 5.29 and W56 5spds, I pulled that grade at 81 MPH in 5th gear with the throttle totally pegged. He was like "WHAT?! I HAVE TO USE 3RD GEAR!"
This comparison seems pretty extreme, so let me clarify some points first. The 3RZ has a strong power band that starts at or around 3,100 RPM. So if I can keep 5th gear above 3,100~ish RPM, then I can pull this grade in 5th gear. But if I let off just slightly, then I have to pull 4th gear, which the 3RZ can do in its sleep.
Knowing that 4th gear is not a challenge for the 3RZ, I asked him how his 2RZ can pull it in 4th gear, and he said not well at all: It won't hold a constant RPM.
So the additional 8 HP and 17 ft-lbs TQ makes a huge difference.
I would LOVE to cruise alongside a 2RZ truck similar to mine and do some speed pulls to see how fast I can walk away from a 2RZ.
BigMike
