P0136 is Bank 1, Sensor 2, so that is going to be downstream of the cat (sensor 2's are always downstream). If a 4-banger, there is only one, but if it's a 6-banger, I think there would be two possibilities (one per bank). It'll be a relatively inexpensive 2-wire unheated sensor.
Part should be in the neighborhood of 50 bucks for one with the OEM connector, about 10 bucks less for a "universal". I'd go with an aftermarket Denso O2 sensor. If you're OK with soldering, you can save a small amount buying a universal Denso, snipping your leads under the car, and soldering the new sensor onto the old wires. Saves you having to get to the connector, too (which, IIRC, is under the driver's seat in the 1996 rigs). Otherwise, get one with the correct connector. Rockauto has decent prices -- just look up based on your year and engine.
As for DIY, this isn't awful. If you live in the rust belt, removing the old sensor is going to be difficult, since it's probably held on by rusted lumps. Then, you have to disconnect the sensor inside the car. pull the cable from underneath, and then snake the new cable up into place. Nothing technical, but I probably wouldn't want my kids listening to me while I did it
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