Thank you for using just answer.
Removing and installing the transfer case as I am sure you know is relatively easy. Simply removing the driveshafts, the electrical connections and unbolting the transfer case from the transmission can be done with simple tools and little experience.
I would not recommend a used unit unless it is coming off a newer model GX that was produced after the issue was resolved. If you are going to attempt tear down and overhaul yourself I would reommend you get a subscription to www.techinfo.toyota.com for access to detailed instructions on the transfer case. Not the easiest job to do, I as an experienced technician can do this in about half a day.