For preventative maintenance, you could reference this thread. Lots of helpful info:
http://www.toyota-4runner.org/3rd-gen-t4rs/221201-3rd-gen-4runner-preventative-maintenance.html
For parts to replace as preventative maintenance, here's the two big ones:
1)
Lower Ball Joints - known to fail and could cause significant damage and/or cause you to crash when they fail.
2)
Radiator - The reason is for the possibility of the trans cooler in the bottom of the radiator cracking, allowing the the ATF and Engine Coolant to mix (aka Pink Milkshake) and then that contaminated ATF makes it's way back to the trans and screws up the transmission. Some buy a new radiator and call it good, some buy a new radiator and also add an external trans cooler ran in-series with the radiator cooler, some buy a new radiator and an external trans cooler and bypass the radiator cooler and finally some just buy an external cooler, bypass the radiator cooler altogether and cap off the cooler nipples by running a hose between the two.
If you enjoy wrenching or would like to learn, here's a list a videos that could help you along the way with your DIY mechanic needs:
http://www.toyota-4runner.org/3rd-gen-t4rs/217550-3rd-gen-toyota-4runner-diy-videos.html
This is an awesome forum and you made a great choice coming here for help. Good luck and don't hesitate to ask questions.
Timmy the Toolman