When i replaced both of my O2 sensors it took me about 2 hours from start to finish because all 4 mounting nuts on my sensors were eaten away with corrosion, so i had to drop the head pipe and cats to remove them. You'll need to jack up the front end of your truck, and place it on jack stands so you've got room to get underneath it. I'd recommend going with the Toyota O2 sensor kit, (P/N 89465-60110) it not only comes with the sensors, but the gaskets, and nuts too. When i replaced my sensors the Toyota kit, it was less money then buying aftermarket sensors. If you look at the Toyota service procedure linked below, and check out pages 7 to 9 it will walk you through the steps to replace both of your O2 sensors. Plus it includes all the part numbers, and torque settings you'll need to complete the job.
http://www.risingsun4x4club.org/uploads/Snowrun/1994 fzj80/1994 Repair manual/Service bulletin/T-CP-SO2-0162-W.pdf