Speak to your state lawyer about this matter. If that doesn't work, then speak to a local federal lawyer. It sounds like you've already gotten some good advice and you need to verify it with a local lawyer who you can give all the facts.
The federal statute of limitations requires that formal charges (an indictment) must be returned by a federal grand jury within five (5) years after the date that the offense was alleged to have been committed, or in the case of a conpsiracy, within five (5) years after the last "overt act" in furtherance of the conspiracy was committed (by any co-conspirator); the conspiracy achieved its goal; or the withdrawal (legal term) by the accused from the conspiracy. Alleged drug crimes may be prosecuted by the state government, the federal government, or both. Dual prosecutions are possible, but rare, based on most state and the federal government's informal policies, and the law in some states. While a plea of guilty in federal court will avoid a state prosecution in many states, a plea of guilty in state court will never avoid a prosecution in federal court if the US Attorney is inclined to prosecute you.
-Joshua Sabert Lowther, Esq.