Why don't I get a split morph option under Body?   This was a tedious morph to make and I don't look forward to redoing the whole procedure again.

 I don't know what "PML" stands for, so I don't know what it does. But, it sounds as if you created a Full Body Morph and want to split that "Morph" while in the main Body set of dials. There's really no morph there to split. The FBM dial for your morph is just a dependency dial that controls the morphs in the different groups. (That's how I see it, at any rate. :) )
When you're "dialing 1" in each group, you're just activating the morph in that group. If you're having to do that independently of some master parameter or dependency, I agree that is unacceptable. :) Just create a Full Body Morph for the Left and Right sides of your character. To do that easily, just make sure everything is zero'd (all rotations, axis and scale settings in your character) and then be sure the morphs in each group are labeled as they need to be so they can be set up easily. For instance, all the morphs for the left side might be "Left Armpit Fix." That would be only the morphs you want activated for the left side. Name all of those associated morphs the same. Then, do the same for the morphs you want to activate on the right, naming them something like "Right Armpit Fix." (You'll have to reload them into the groups under that name.)

Once done, dial all the ones you want for the Left Armpit Fix group to "1", then go into your "Figure" dropdown, in the top menu, and "Create Full Body Morph." Name it exactly the same as the morph targets are named for that morph = "Left Armpit Fix." Zero those individual group morphs out and then go to the Body panel and "1" for the FBM you created, to test it. If all is well, proceed to make the FBM for the right-hand morphs.

Because this is a "fix" that is only applied when the joint is moved, it's easier to work with if you don't have to do anything. :) Setting up to be dependent upon the rotation of a joint is probably the best idea, as long as the fix morph is good and doesn't, itself, require any tweaking. Instructions for how to do that are in the Poser manual.