4wd problem

My wife's 2012 raptor has some sort of problem hopefully you guys can help! When you turn the switch to 4wd instantly says service 4wd system. I pulled the vacummn line off top solenoid and has vacummn so I went down to the passenger side wheel and still has vacummn there as well but the drivers side doesn't. Could that be why it says it and if so what would cause this??? Figured a cracked line but you'd think the whole system would loose pressure. All help greatly appreciated