Differential fluid change?

The OE number is 3.5 quarts for full, but normally the better method is to remove the fill plug and check the level there. Full is level with the hole to 5mm below. I generally check it cold because the oil will expand as it warms up, so if you check it warm you may have some leaking out if it was up to the plug cold.

75-90 will be okay, they call for 80-90 for ones that may be operated in the cold so the 75-90 will be fine. As long as the truck was level when you checked it you should be okay. LS additive won't hurt if it doesn't use it, just makes it stink more.

The only reason it might go lower would be if it leaks. Driving around the oil will get warmer and expand from the heat, so if you were to check it on a level surface with it cold and it was right at the fill plug, then you checked it after a long trip with it still hot, it would likely leak out a bit due to the expansion. But if you check it cold it would still be level with the plug. As long as the truck was level when you checked it and the oil is at the fill plug it will be fine.

You can use Dexron 2 or 3 but I would opt for 3 as it's the newer spec. As you want to replace it I would go for a synthetic Dex 3. Then to change it, use a turkey baster or suction device to remove as much fluid from the reservoir as possible. Now disconnect the power steering return line from the side of the reservoir and cap the fitting on the reservoir. Add a length of hose to the return line and aim it into a bucket. Now fill the reservoir with new fluid. Keep adding fluid while you have someone else start the engine and shut it off while you keep the reservoir topped up. Have them also turn the steering wheel from side to side to flush out as much old fluid as possible. Just be sure to keep the reservoir full.
When you see new fluid going into the bucket stop, use the suction device to remove the fluid from the reservoir and reconnect the return line. Top up the reservoir and start the engine, turn the wheel to each side until you hit the lock. Repeat turning the wheel a few times. You are finished.

Sounds more like it's a mix. The stock fluid would be red and most of the stuff tagged as power steering fluid is clear or red. Tan brown sounds like moisture in old fluid.
But it's your choice.