You're in the unenviable position of being significantly upside down - meaning of course that you owe quite a bit more on the loan than the truck is worth. You have the following options...
1. Roll the $10,000 that you owe over the value into the next car loan. This is often done, but not really recommended as in the long run, you're going to end up owing a LOT more than whatever you buy next is worth. If you're trying to buy a $6K used car, you might not be able to do it at all unless you have stellar credit - you'll owe $16,000 on a $6000 car and $6000 cars aren't really considered decent collateral for a car loan. You could buy something new, and then you're really headed into the hole.
2. Sell the truck privately - you'll probably get $3-5K more than your trade value. I never trade my cars as I just don't wan to give away $5000 for the convenience of not selling my own car. That said, you're going to still end up upside down and you'll have the outstanding balance to deal with - see if your bank will cover you on it. Suppose you get $25K for the truck - and then buy your $6K used car - now you're $11K owing, so "only" $5000 upside down. Better than $10,000 - but still not great.
3. Keep the truck and keep making payments until it's either paid off or you are no long under-water on the loan.
I get that the trick probably sucks gas - but if possible the least expensive way for you is going to be to keep it for some time yet - is there some other expense you can trim? I know people with $700 car loans that complain about them but could find $300-400/month in other expenses they could cut until they pay down the car loan.
In the future, to avoid this again, think about only financing over shorter terms (say 60 months) and try to put money down to keep from getting upside down. Also, it can be tough, but think about that with whatever you buy, you want to own it for the term of the loan.
