Scott,
Follow up to the cracking issue.....Most of the cracking that I'm familiar with on older 99's were at either side of the tang junction. Some believe this was caused from the stock bolt loosening and the action shifting/hitting the inletting when fired. The kind of split you describe sounds more like what Jim explained from excess oil seeping into end grain of the wood and weakening it. Also, obviously, if it had been dropped that could explain it.
Yes, by all means you want to get the split stabilized, by pinning and gluing w/ Acra-Glas. A qualified gunsmith should be able to do the job.
Jim