My now ex wife cashed three checks that were solely in my name.. we were still married but separate and she forged my signature without my knowledge and deposited almost 2500 dollars into her bank account. This was about 9 months ago and I just barely found out. Can I still prosecute her for forgery?

You can't prosecute her but the state can. Contact your bank and inform them that you never signed the checks.

Of course, it's only illegal if her name wasn't on the checking account.

Yes. The DA needs to be informed of her actions if she was not on the account. She could spend 6 months in jail for her theft. Bring along the original forged check to the DA's office.

After re-reading the OP's original question, I'm wondering if we misunderstood. The question reads:
"... cashed three checks that were solely in my name."

She's not allowed to sign for a check that's not made out to her, so I suppose it really has no bearing whether she was on the account or not. She signed something that wasn't hers.

You didn't ask about how to recover the money you lost, but perhaps this would help: If your divorce is not yet final you should include the value of what she took from you in the calculations for splitting assets. If the divorce is already final, you can sue your ex in small claims court to recover the money.