You might want to take a look at your loan to see if it uses a calculation method called "the Rule of 78" or "Sum of the digits" to compute your interest charges. If this method is used then it means that you may not have the equity you expected in your truck. If this method or other similar methods were used it means you have already paid much of the loan's total interest charge and consequently very little equity, by comparison, was accumulated.
Laws in your area might not allow the use of these methods but it would be worth it to check it out before proceeding with your downsizing plans.
I have not had an auto/truck loan since I discovered that these methods were being used to help lenders earn their interest early in the life of the loan.