Based on the wiring diagram it seems to engage that speed switch solenoid based on engine speed. I'm not sure what that solenoid does, but I would guess it has to do with idle speed or transmission kickdown.
I just did a similar thing to my truck. I already had the 14 FF on the rear, but finally upgraded my front to the 8 lug.
I initially ordered some wheels from 4 wheel parts, but they didn't actually have any in stock and gave me a couple month back order time frame.
I ended up cancelling and...
Setting up the carrier is fairly complex task. If you don't get the backlash and preloads set correctly you'll ruin your gears in no time. Here is a nice guide from US Gear that I think is very thorough.
I actually happen to have a partly disassembled 10-bolt laying in my driveway. It's all in surprisingly good shape except the broken u-joints on the passenger side. I'd be willing to sell you any part of it. I'm really not sure what the going rates are for these.