The shocks are the thing that limits suspension travel anyway. So I guess the question about shock length would be if you jack the truck from the frame how much does the shock unload before the axle comes up with it? If the answer is very little, it's not the amount of force, but the repetitive force, and you may be dealing with odd forces like the leaf pushing the axle downward, effectively making the axle weight higher.
If you're changing them anyway it is a none issue. But it would be nice to know why your frame is cracking, if there is no apparent cause, then I'd be worried about it being defective, and getting more and more cracks in the future.