The only reliable way to know what a car is worth is to test the market with a listing. You can get a starting point by looking at kbb, or browsing through autotrader and ebay listings. But IME, autotrader listings, most of which are dealers, are high. Those are list prices, not selling prices.
So list your truck, as you have, and if you don't get any nibbles, that's the market telling you you're priced too high. So drop it in increments of $500 every couple weeks, and when people start calling you know you're getting close.
The only trucks I know with values that high for the mileage are Toyota. Last year DH sold his 2002 Tacoma with 220K. Kbb said $8k, so he started there. Eventually sold when he got to $6500.
I also sometimes find it helpful to turn the tables and ask, "what price would make me want to jump on this as a great deal?" start a bit higher than that.