I have no idea how old you are, or your background...... but do you really have to ask if running fuel that old thu a diesel engine is a good idea?
In case your wondering, the answer is No.
^^ a little more direct than what i was going to say but no, you should not run diesel thats been sitting in the truck for 18 months. Diesel likes water, your truck does not like water. Water is also denser than diesel, so it will settle to the bottom of the tank if there is enough of it and you'll have a real bad day then. You should absolutely siphon out the tank, it would probably be a good idea to change all the other fluids and filters on the truck as well. at minimum change the oil and oil filter as well as the fuel filter. Bonus tasks would be to change the brake fluid, steering fluid, diff oil, well..... probably everything but coolant should be changed, and coolant depends on how gross it looks.
The problem with old fluids is that under normal use, they get hot, usually hot enough to boil off water, but at least hot enough to help vaporize the water. This prevents buildups of water in the oil and other fluids. When the truck sits, this allows water to accumulate without getting cooked off and you start running into issues.
You also probably want to replace the batteries... they are almost certainly shot.
Although, I had a 6.2 liter diesel suburban that I filled up before I left to work overseas. It was in an unheated building in north Texas. When I came back to get it 7 years later it only had about 1/2 tank left. I did drain a little fuel out of it and it was real oily. I was just going to turn it over with starter till I got oil pressure but I couldn’t remember which wire it was, there were two on the ip. I obviously disconnected the wrong one because after about 5 seconds of cranking it fired off. Smoked up the hanger pretty good but ran fine. After about 15 mins of idling I drove it to nearest store, about 2 miles and filled it up with fresh.
But remember the older diesels would run on straight clean vegetable oil.
yea... those old diesels would run on oil soaked gravel if you could get it into the engine... i would not flow that through a modern diesel though.