91 XJ: front driveshaft double-cardan lubrication

If I jack up both front tires of my cherokee, apply the parking brake and
put the auto-trans in neutral and leave the xfer case in 4WD will I be able
to freely rotate the front driveshaft so I can get to all the zerk grease
fittings?

Also, what's the double cardan joint ball for? In looking at the exploded
diagram of the double cardan joint in the front driveshaft (right next to
the xfer case) there is a ball fitting that looks like its got a grease
fitting. What I'm wondering is why you need this spring-ball at all. If the
spiders are handling all the load from transferring power from the
driveshaft through the double cardan joint then what is the point of the
ball that goes between the spider and the center yoke in the double cardan
joint?

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