Oxygen sensor socket or wrench. Try a search here, since I recall a few others posting about what they used. One poster had a photo of a slim/thin box wrench, another mention a short ratchet....to assist in loosening the sensors, you may want to consider using some penetrating oil on them beforehand. Of course, you'll have to raise the car to get to them.
There are 4 of them on the 4.6L models...you'll find them in the exhaust line: one before the catalytic stuff, and one after, on each side.
Do you recall what code(s) was (were) scanned, or are you just going to replace all four, and see what happens?
From the 1994 Service Manual:
Heated Oxygen Sensor (HO2S)
Removal and Installation
NOTE: If excessive force is needed to remove heated oxygen sensor (HO2S) (9F472) , lubricate heated oxygen sensor with penetrating oil prior to removal.
--Disconnect battery ground cable (14301) .
--Disconnect heated oxygen sensors from engine control sensor wiring (12A581) .
--Raise vehicle on hoist. Refer to Section 00-02.
--Remove heated oxygen sensors from dual converter Y pipe (5F250) using Oxygen Sensor Wrench T94L-9472-A.
--To install, reverse Removal procedure. Tighten heated oxygen sensors to 36-46 Nm (26-34 lb-ft).