The balance shaft is for the 4X4s only. I put one without a balance shaft in a 4x4 Explorer and didn't notice anything. Who knows why it's there, but there must have been some trend in the 4x4 drivetrains that caused them to use it.
I attached a few more 4.0 things. The pdfs were posted a long time ago by another member. He used to identify himself when I posted them--hopefully he's still around.
And this is my superimposition of the Honda 3.5 V-tec on the 4.0 curves. I made that when I was betting Dustin it would out tow some damn thing or another. In this case, you can see where the intake manifold opens up the plenum and then the valve timing shifts and the 4.0s are diving for the deck. My premis before is that the 3.5 Vtek will lag through first gear by a little until 2,000rpm on the pushrod motor and by up 20ft# until 3,500# on the SOHC--then it will never lag again--it will be seriously laying a whopping on those 4.0s because it's 2nd gear recovery point is 3,500rpm and it keeps getting higher. This is the type of engine that will end up in the next Ford compact truck and everyone is going to cry. There's no reason for it though. it will do the job easily and have an extra 50hp when you want to have fun with it.