if you can get it for free, it's definitely a bargain! They're about $400+ retail, brand new.
I got mine for $35 off craigslist, locally... and that was a steal.
If you don't already have the mounting pins on your truck (the FX4 Level II's come with them, the rest of the models only have them if the option was ordered with the truck), you'll need to pick some up from Ford or from one of the online Ford parts dealers. I ordered mine online and had 'em a few days later -- I think they ran about $7.50 each... the local dealership wanted $15 each for them.
In the back of your bed, you should see three holes on each side. The pin screws into the center hole on each side. The top and bottom holes are for the retaining pins on the extender to pop into to stabilize the extender in position (open or closed).
2001 Ranger XLT 4.0L SOHC
Silver Frost w/ Dark Graphite interior
Summit 3" Body Lift
31x10.50 A/T tires
Explorer overhead console
Explorer auto-dim mirror
Aux1 & Aux2 inputs on factory AM/FM/CD/Cassette
Door-dinger silencer button
Seamless retractable roof and passenger-side ejection seat with steering wheel mounted controls and safety interlock