What's the cost to replace the tranfer case

Hello, I'm sorry - but you definitely need a professional assessment for this. It's impossible to know without a hands on inspection for the oil leak and an evaluation which results in knowing what is needed. The whole case, I would imagine, will not need to be replaced. Either way, if so, a repair shop near you would provide that estimate also. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops