I still have a check from taxes 2018 can I still cash it or should I ask the IRS for a replacement due to the fact its 2 years old.

 Under federal law, U.S. Department of Treasury checks must be deposited or cashed within one year from the issue date. Although you cannot legally cash your expired Treasury check, you still have a right to receive the funds. Do not destroy or throw away the check. You must follow the agency's reissue process to receive a new one.

Call IRS at 800-829-1954 (toll-free) and either use the automated system or speak with an agent.