Could be gearbox or diffs....
the 4.0 THOR is on a par with the 4.6 GEMS, the 4.0 is a bit sluggish compared to the 4.6 with most of the difference in initial acceleration off the line with the extra torque from the bigger capacity, but liveable with aslong as you don't need to drive like a lunatic!
Having had both, when I had the 4.0, I was always wanting to 'want' a little more from it, if you see what I mean....if I had never driven the 4.6 I wouldn't notice nor want from it.....but as I have had 4.6 previous...the 4.0 was (disappointing is the wrong word) just a bit lacking...but not in a bad way...just a .... a.... can't find the word.....