You can file to lift conditions by your self, by filing I-751 you have to request a waiver from the joint filing requirement.
From the I-751 instructions:
QUOTE
If you are still married, the petition should be filed jointly by you and the spouse through whom you obtained conditional status.However, you may apply for a waiver of this joint filing requirement if:
- You entered the marriage in good faith, but your spouse subsequently died;
- You entered the marriage in good faith, but the marriage was later terminated due to divorce or annulment;
- You entered the marriage in good faith and have remained married, but have been battered or subjected to extreme cruelty by your U.S. citizen or permanent resident spouse; or
- The termination of your status and removal would result in extreme hardship.
Note you can file to lift conditions at any time before the expiration of the card, the waiver from filing jointly allows immediate filing.
On the form just check "d" in part 2 to request the waiver.