1999 4Runner Maintenance Guide definitely specifies timing belt change for 4Runners equiped with 6 cylinder engine. Here's the list of maintenance from the guide:
Replace engine air filter
Replace engine oil and oil filter
Replace lmited-slip differential oil
Replace non-platinum spark plugs
Replace timing belt
Lubricate propeller shaft and re-torque bolts (4wd)
Re-torque drive shaft flange bolts
Inspect: Ball joints and dust covers, Brake lines and hoses, Brake linings/drums and brake pads/discs, Differential oil, Drive belts, Drive shaft boots, Fuel lines and connections, fuel tank band and fuel tank vapor vent system hoses, Exhaust pipes and mountings, Fuel tank cap gasket, Steering gear box, Steering linkage, Transfer case oil (4WD models), Transmission fluid or oil
Additional maintnance items for special operating conditions:
Inspect nuts and bolts on body and chassis
Replace differential oil (including limited slip)
Replace transfer case oil (4wd)
Rplace transmission fluid and oil