2000 F150 4x4 transmissions

For 350 you can buy a rebuild kit and all the tools you need to rebuild it. Usually when a transmission fails its because the clutches in the trans are worn down, or you have too much clearance

If you purchase something out of a junk yard, you do not know what you are getting or how long it will last. If you can pull the thing out of the vehicle yourself, try to rebuilt it yourself.

I would not rebuild it yourself unless you are a VERY accomplished mechanic that has experience on automatic transmissions. Most transmission shops can't get it right.