I’ll be going out on maternity leave later this year. Our employee handbook states that I am eligible for 8 weeks paid leave (“full pay will be extended for eight weeks for parental leave”), and I can combine it with vacation time. Available FMLA must be used concurrently with this policy.

I am paid a base salary plus profit share based on the profitability of the department - I am the head of the department. Profit share is 50% of my compensation package. My boss is telling me that since I'm not at work I will not receive the profit share, during either the 8 weeks paid maternity leave, or the 4 weeks vacation. Profit share is typically accrued during vacation. There have been instances of other department heads going on maternity leave and receiving their commissions/profit share, and a more junior person going on unpaid leave and receiving partial commissions/profit share while out.

If the handbook states "Full Pay" during maternity leave, should I be entitled to my profit share during this time?
Is it legal for them to remove the profit share during the vacation taken in conjunction with my leave?

Since they don't have to pay you at all, it is up to the company what they do pay. You can ask why others were paid the full amount and you will not be but unless they are using a legally protected characteristic to decide who gets the full pay and who only gets the base, it is legal.

To me that just does not sound right. You should receive your full pay if that is what the handbook states. I would ask about it in a manner that is not threatening. I have seen people take similar things to court and win. But of course on the down side of taking your employer to court would be the changed work atmosphere that would come about from such actions. You might want to double check and make sure a change to the policy has not been put into action . It is possible they have and perhaps you did not receive the update through some odd incident. (And I do work in a court room and the written documentation means a lot.)