If you cannot sell it for more than what they offer, plus the tax credit, just trade it in. The other option is to see what Carmax will give you when selling it to them. Their offer is almost always higher than a typical dealer offer, unless your truck is highly modified.
I always sell mine outright as it is pretty easy to get more than 6.25% (Texas sales tax) over the trade in offer. Plus, you really are not dealing on an equal footing when you trade. They can offer a seemingly bigger discount than other dealers, but low-ball on your trade to make up the difference. The only way to really compare dealer offers is to do it without a trade in the picture.
Dealers know it is a hassle to sell your existing vehicle when buying new, so they use that to their advantage when making their low trade-in offer.