You should have Bill of sale or I guess in this case a Bill of Trade, it would state A is trading X to Person B for Z....listing names and address, dates, vehicle make, model year, vin, and a clause that states the trade is AS IS for both vehicles. But even if you don't have that, sales/trades between private parties are generally considered AS IS unless there is some written warranty.
So you should be fine. But if there is any legal action take that seriously and consult an attorney.
Also note this a common scam buy/trade for a vehicle, claim some problem and then try to get money back from the seller. Pressure them, call a lot, claim you will sue, etc.
Or he is a nit wit and drove off road and broke something.
Be polite, firm, and very clear. Tell the other party that the trade was AS IS and that you were not aware of any drive shaft issues. At this time I consider our business concluded and based on the fact I have explained this a few time, I would request that you do not contact me after this call. DO NOT get into a back and forth or any debate, or make any threats, just say, Our business is concluded, please do not contact me after this. That it, the guy knows what that means. If you get an email or text, reply with the short note....On March 22 at 2pm you called me, at that time I made a clear request to end any further contact. Please delete my number from you phone and do not contact again.
If it continues hire an attorney to call him. Yes that will cost you money, but these things happen.