My Linjcoln MKX has a slight vibration at speed 25 to 40 MPH. What could be the cause.

Speed sensitive vibration is usually tire balance. A failing wheel bearing or separated tire can do it too but wheel/tire balance is usually the culprit. You might consider having your alignment and chassis checked while you're having your tires rotated. Don't cheap this one if you want a thorough inspection. Lots of shops will do an alignment even through the entire suspension system is wiped out (PepBoys etc.) A reputable shop/technician will tell you what is wrong with your vehicle and give you an estimate before trying to align a worn out front end. (the concern with alignment and chassis is in reference to the uneven tire wear).

my lincoln mkx vibrate only when t slidely curb and go off .and also some time on highway when i press gas other time run smoth dont know why do like that .some my friend says it is transmission problem i need to fix it please help me.