Can I force a lienholder to repossess my vehicle?

No, you cannot force a lienholder to repossess the car.  You may want to call someone who hauls away Junk Cars -- they usually do not require a title.  To be safe, you could contact the lienholder to warn them that the collateral is going to be hauled away, but it is probably better to just have the car hauled away as junk.  If you haven't paid in six years and haven't been sued, then you may be past the Statute of Limitations in Florida (usually 5 years on a written contract).