Originally Posted by Bad Luck

Welcome to the forum. The two biggest failure points are the transmission cooler located in the radiator and the lower ball joints. When either of those fail it gets very expensive.

If your radiator plastic is brown or the coolant looks gross then I would suggest replacing your radiator and radiator cap. Generally recommended service interval for an OEM/Denso radiator is 10 years/100k miles. When the transmission cooler in the radiator fails it causes coolant to get into your transmission and ruins the transmission unless caught very quickly.

If you don't know when the lower ball joints were replaced (if they have been replaced) then I would recommend replacing the lower ball joints and lower ball joint bolts with OEM only. To accurately check your lower ball joints for wear is a whole procedure and requires a dial indicator. It's easier just to err on the side of caution and replace them if they are old or unknown. Generally recommended service interval is 10 years/100k miles. When a lower ball joint failure happens it gets very ugly and dangerous very quickly. site:toyota-4Runner.org/3rd-gen-t4rs/ lower ball joint failure - Google Search

Timing belt should also be checked/replaced around 100k miles. You can pull the 10mm bolts out of the upper timing cover to look at your timing belt condition. If it's cracked then it's time for a replacement timing belt/bearings/water pump/thermostat.