906s are aftermarket springs made by OME...they can be purchased from many different off-road stores. The 5100s are adjustable, so you can adjust not only the height the spring sits at, but this also can affect the cushion of the ride...makes for a very capable shock that lets you dial in a ride you like. You could also get the 4600s, but you lose the ability to adjust and the heighth increase...they are comfortable ride from what I've read.
For the rear, I suggested the 906s because they give you between 1.5 - 2" of lift. They are good if you want to carry a medium load, as they won't sag too bad. They also give a good stance when paired with the front setup I recommended. The 4600s will work with these springs, but that is about as high as you can go with them.
The rear is easily done at home, much easier with a friend or two. The fronts are easy, but you either need a spring compressor or you can take it to a local shop and have them put together for a fee. The whole thing shouldn't take more than 4 or 5 hours, depending on how rusted eveything is.
Just google "suspension install 3rd gen 4runner" and a few will pop up.