Engine vibration at 45 mph and above

I recently purchased an 04 Tacoma Ext. Cab Sr5 V6. Absolutely gorgeous truck with only 79,000miles on it. I test drove it around town and had no problems. But now I'm noticing some engine vibrations when I hit 45-50mph. I also notice that the shift knob has a tendency to want to slide back especially at higher speeds (it doesn't but pulls that way). I haven't gotten it checked out yet but am beginning to worry that it's a transmission problem. Has anyone else had this problem? Thanks for your help!