It was available with the 2.7 (3RZ) automatic in Prerunner (6-lug) version and the 3.4 (5VZ) automatic in either Prerunner or 4WD form (no manual trans in a DC).
The double cabs (01-04) were all 6-lug and therefore only had those 2 options, and the 5 lug mini which got the 2.4 (3RZ) wasn't offered with a 5VZ except in S-Runner configuration.
Simply stated, either the engine was swapped, or the seller is mistaken. You won't find a 2RZ in a 6 lug truck unless it has been swapped... which isn't that big a deal, since they are in essence the same engine except for stroke.
I feel the RZ's were some of the best engines Toyota made, but a bit underpowered (by American standards) for that large a platform, especially if outfitted for expedition use.