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Rainbird and its competitors have a system that isn't too expensive and it's easy to set up. It consist of a 1/2 inch supply line, 1/4 inch drip lines with nozzles that have different flow rates, and a pressure regulator.
You can buy a kit that really won't help you much, or you can buy the components individually.
You just attach a water supply to the pressure regulator, connect the regulator to the 1/2 line, roll the 1/2 line out along the line of trees (or thread it through the trees to keep it off the ground), plug a 1/4 line into it at each tree, and plug a nozzle into the end of the 1/4 line. It's very easy, but be sure to replace the crappy plastic connectors they supply with the pressure regulator with power clamps.

The connectors for the 1/4 line come with a tool that lets you insert them into the 1/2 line at any place, so you can customize it for your needs. For those big trees I would use 2 gpm nozzles. Somewhere along the line plug in an extra 1/4" line with a nozzle and put the end of it in a container to catch the water. That way you can monitor the performance of the system and adjust the amount of time you let it run.
This is the system I use on my tomatoes, which are 6 feet apart.