When cruising in 5th gear at like 3k rmps and I floor it, then car shakes or vibrates slightly. Do I need an alignment or something this is very annoying..

 

i'd check tire/wheel balance and runout on the rims, and brakes. thats pretty cheap to do, most tire stores will do it for under 10 if not free. then i'd look at alignment, cause it never hurts to re-align. then i'd also look at the inner axle joint, when those go bad you'll vibrate under acceleration or deceleration, but usually its in all gears. when vibration occurs at speed, i'd say its improper tire inflation or wheel out of balance. have fun.