I would recommended before selling your truck that you do a calculation of how much you will get for the car minus the price of the old truck you will buy and additional maintenance cost you will incur.
I bought cash a Toyota Camry last year. Before moving to California I swore never to buy a new car because I could buy a one or two year old car and not pay the premium. But when I moved here and had to buy a car, all the old cars were really expensive considering the mileage they had. Cars were selling for $14K with 50K-60K. I manage to negotiate the price to the dealer and paid $18K for a brand new car. I figured I can put the 50K miles to my new car and that will cost me $4K only.
I also look older cars (10years old), but they had 200K-300K miles. Chances I will need to put money on them are very high so that did not work out for me either.
You said you can get $15K but how much do you think you will put into maintaining the car in the next 5 years? $3-$5K maybe? Then is $10K extra money better for you than having a nicer car? This is your call.
You mentioned if this is a good time to spend money. I think the problem is not today or tomorrow or yesterday but if you are spending beyond your means (like using HELOCs to pay for luxury items). This is clearly not the case for you. So the best time for you to spend or not your money really depends on what your long term plans are and your financial situation. If you can afford it and have a plan for your future then I think you are ok. If you used all your money to pay for these 2 items, then you may want to analyze how this affects for future plans.
As novice1027 said, There must be a balance in our lives. I think about the future but I also put a weight on what makes me happy today.
