With excess grease in there, it would border on hydraulic locking the shaft and putting extreme strain on the transfer case, diff inputs, or u-joints. Not just increased spring rate. I wouldn't presume taking out the zerk and putting AHC low would be enough. The suspension can and does compress even lower than low position in use. Hope he didn't do much or any driving like this, as with that many pumps was probably jacking up the car by the driveshaft.
Probably best to disassemble or take the zerks off the drive shaft for some number of miles and let it clear most of the excess grease.
My comment about spring rate was in response to
@East100Cruiser noting difficulty removing his front driveshaft. I didn't specifically say it, but yes turning the slip yoke into a hydraulic cylinder is exactly what would make the extreme forces I referred to earlier, and is the big risk with the condition OP posted trying to resolve.
Zerk out, AHC low, will remove enough grease to make the car safe to drive for normal use. Then it is lifted back up, and the volume in the yoke displaced by the male portion of the splined section will fill with air. As it is driven further the excess will work it's way out through centrifugal force spreading it all out into the splines, then normal driving compressing and extending the driveshaft in a smaller range causing shear stress (in the fluid) that works the grease out slowly, as well as a little air-spring driven positive displacement pump on suspension cycling to push grease out as well. This will have happened by the time the suspension is fully compressed, unless the truck was being prepped for an off-road trip. In that event.. removing the zerk and driving wouldn't hurt. I don't think it's necessary though.
For those wondering how grease will "flow" like a fluid into and down the splines, assuming the splined section of our driveshaft is about 1.5", at 75mph there are roughly 225 times the force of gravity. that will make a thick fluid, even non-newtonian as our driveline grease is, flow really well.
The air-spring effect, hydraulic lock, whatever it is called, is why I've always been uncomfortable with toyota's published method of pumping grease in until you see the shaft move. They are basically instructing you to push grease in to the point that the pressure in the slip yoke turns the driveshaft into a ram, spreading it and putting force on the transfer output. This can't be a small amount of force.. plus.. the force required to move things will depend on whether the shaft is straight like our front one, or flexed like the rear. Or the height of the rear suspension. As you mention, yes, you can even lift the vehicle by pressurizing the driveshaft. To me their instructions are too subjective, so I do what I and thousands of others have and throw an amount in that feels right, and keep driving. Toyota shafts almost never wear out the slip section, so it must be working.
