Mercury

I bought a used (obviously) 1999 mercury mountaineer. i love it, but on the highway, at about 55 mph, it starts shaking. along with that, the tire noise it always loud, no matter the speed. does that sound like an engine issue, or a tire issue?

Sounds like a tire issue. they may need to be balanced or you could have one with a broken belt which would explain the vibration or badly worn tires accounting for the noise at lower speeds. in any case you should take it to a shop you trust and have it checked out.