I run napa axles in my car and havent had an issue, but thats not to say your new axle you bought is a bad new part.
If it was me, I would replace the other side aswell, just to see if that one is the cause. Then if the problem is still there, I would get a new one for the drivers side.
Also make sure your motor mounts arent blown out or smashed from age, that can throw off the axle geometry.