If I had a four wheeler accident 5 or 6 years ago but didn't file because the driver was a friend and now my back is getting worse, is there legally anthing I can do?

Probably not.  All states have a statute of limitations for filing a personal injury claim.  A statute of limitations is a time frame in which an individual has to pursue a claim after suffering an injury.  Most states have a two year statute of limitations for personal injury.  At this point, if your accident was 5 or 6 years ago, the statute has likely run and you have lost your ability to pursue your claim.  To be sure, you need to check the law for the state in which you suffered your accident.