NP231 Command-Trac Question

I've got a 90 XJ(4.0, AX-15, NP231, 188K) and a 98 TJ(2.5, auto, NP231,60K) and I was looking at the front drive shafts on both of them when I noticed something wierd. On the TJ when in 2WD and parked the front driveshaft(CV?) is not engaged but is locked in place but on my XJ when it's parked and in 2WD the front driveshaft spins freely. They both have NP231's, is what I'm noticing on my XJ normal?

If the 90 has a vacuum disco axle it will let the front drive shaft turn in 2wd mode.