The 4.56 gears will cost a litle more but no huge price difference.
I would double check with the guy that siad he would install them for $100
The rear is not too hard but the front is. To do the front set you have to completely remove the fromt differential for the Explorer.
This means removing the hub bearing assemblies, steering knuckles,axle shafts, & sway bar links on both sides.
As far as the Jet goes I have heard that they are a waste of money as Explorers are concerned.
Search the Under The Hood Forum for info on chips & tuners.