Vibrates

When you had the tires rotated did you notice if the tires were not worn properly? Meaning one or two were more worn then the others? I am assuming this vehicle is an All Wheel Drive vehicle as most of them are. If this is the case then the tires need to be replaced all at the same time. If you have different sized tires then this will cause vibrations in the drivetrain right around 40 MPH or just under during acceleration.

If this is not the case then more than likely you have a tire issue that was on the rear and now that it is on the front, you just feel it in the steering.

As far it being safe, based on what I am hearing I don't think it is a safety issue but if you are nervous about it then just have it checked out as soon as possible. It is easy for me to say it is safe but I am just some guy on the internet that is reading three sentences. You are driving it and know what will make you comfortable so let's do what ever that is.