If you are filing your 751 asking for a waiver of the requirement to file jointly with your spouse due to divorce or abuse, you can file early (or late). You do not have to wait to file within the 90 day period before your conditional green card expires:
"Filing with a request that the joint filing requirement be waived or individually filed. You may file Form I-751 without your spouse if they are deceased, you are divorced, or you and/or your conditional resident child were battered or subjected to extreme cruelty. You may file this petition at any time after you are granted conditional resident status and before you are removed from the United States."