You can also measure the axle shaft. Light duty rotors fit the 4" shaft and the heavy duty fits the 4 1/2" shaft. I have the 4 1/2 on my 98 ext cab and the bearings were a-3 for outer and a-5 for inner. It could also be as supertrucker1978 has stated.
Yes you can install the head unit now and wait on the other speakers. When you get the amp and subs later you will have to take the head unit back out to hook up the rca cables and amp remote turn on lead wire.
You should have a fitting on the upper, lower ball joints, inner, outer tie rod, pitman to centerlink, idler to centerlink, possibly a fitting in your u-joints on drive shaft if they have been replaced. Not sure about the frequency tho.