Usually the newer vehicles have two oxygen sensors one located in the front near engine on the exhaust manifold or the front pipe and one near the catalytic converter,the back 02 sensor usually lasts twice as long as the front one.

In the USA as it is stated in the owner's manual under the section of "scheduled maintenance" the front o2 sensor is replaced free of charge when the vehicle reaches it's first 80 000 miles regardless of age and numebr of owners many Toy dealerships are unaware of this warranty policy or are reluctant to replace if that's the case contact the Toyota Regional Office for your state and explain your situation where they will give your local Toy dealership 48 hours to comply.
On average the oxygen sensors should be replaced every 6 years or 100 000 miles or so which ever comes first.

Symptons of a bad o2 sensor are poor gas mileage,hesistation in acceleration and worse emmissions,the 02 sensor is designed to function once the vehicle has warmed up which is about after 10 minutes where the thermostat begins to open then the vehicles ECU system enters what they call a "closed loop system" then the exhaust manifold's temperature reaches 400 degree Ferenheit (I think)that where the sensor starts to do it's job,may folks think that the o2 sensor is bad but an exhaust leak can cause excess oxygen to enter the exhaust system thereby fooling the sensor into thinking there is too much oxygen,there are other problems that could cause it to give false reading such as a vaccuum leak.

If you discover that the o2 sensor is bad and it is not under warranty then from my/others experience/research purchase an o2 sensor in the following order:

1)Toyota (NipponDenso now called Denso)
2)Denso aftermarket (Same as original but sold aftermarket)
3)NTK (oxygen sensor division of NGK)
4)Bosch

Available at:
1)www.1sttoyotaparts.com/part_number.html
(they have the lowest prices in the USA & probably the whole world)
2)www.densoaftermarket.com
3)www.ngksparkplugs.com
4)www.bosch.com

Note:Toyota has an organized numbering system which dictates that the first 5 digits of their 10 digit (5 digits-5 digits)part numbers represents the part # and the remaining 5 digits dictate country of origin,color code,trim package and so on not in that order.

NGK has a powerfull oxygen cross reference software which cross references their different part #'s into either a single,double,triple or a quad wire o2 sensors so you may find 20 or different part #'s for your 94 Landcruiser and all you have to do is plug those part #'s into:

www.1sttoyotaparts.com/part_number.html

and see which one is cheaper I think you will find one for as low as $60.The NGK cross reference software is:

home.att.net/~ngksparkplugs/wsb/html/view.cgi-home.html-.html