it is not too difficult if you have the right tools.click on links below for instructions on gear install;
http://www.superstallions.com/tech/drive/gears/gears.html
http://www.louisvilleoffroad.com/Ring-and-Pinion-Installation-sp-9.html
and here is a good thread to read about gear installations click on link below.
http://forums.stangnet.com/showthread.php?t=618253&highlight=gear+install
below is a list of parts you will need to install gears properly these are the exact parts i bought to install mine so i know there good quality....
i bought a used set of ford gears off e-bay and saved approx $125-$150 but before buying you need to contact buyer and ensure he has a return policy and that gears are in good shape with no breaks,crack or chips and pay thru paypal to only paypal verified sellers this is a step most people overlook but if you do as i stated above you will get a good quality part and if you have problems you can return for a refund and if you still have problems e-bay and paypal will help but this happens rarely if you do as i said by contacting seller and asking questions,ensuring they have a return policy and buying only from verified paypal sellers with paypal its worth the savings it was for me click on link below for a set of ford gears for sale...similar to the one i bought but in a 4.10 size.
http://cgi.ebay.com/ebaymotors/410-...8QQihZ004QQcategoryZ33731QQrdZ1QQcmdZViewItem
you will need a master bearing kit like the one in the link below...
http://cgi.ebay.com/ebaymotors/Ford...ryZ33731QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
traction loc rebuild kit like the one in the link below... friction modifier is included with kit...
http://motors.search.ebay.com/8-8-t...atitleZ8Q2e8Q20tractionQ20lokQ20rebuildQ20kit
rear axle shaft bearings and seals (2) you can purchase these at your local parts store...advance,autozone ect...
3.75 pints of hypoid gear lubricant sae 80 or 90...get this at any autoparts store...
and a speedo gear click on link and find your application...
http://motors.search.ebay.com/searc...00&fpos=35905&ftrt=1&ftrv=1&saprclo=&saprchi=