Sell it where as is, where is to an individual you will get more than a trade in. Putting money into transmission will be good if you are going to keep the truck for a long time. If you are going to keep it for a short time, you will never get the money back. As you put more miles and years, the value of the truck only goes down. You will start to have other issues also as parts start to wear. As for me, I drive a vehicle till the wheels fall off or a deal I cannot refuse comes along (that is not too often.). My ranch truck a 98 dodge 2500 has 465,000 miles still going strong. My wife's 2012 Sequoia has 200,000. Both vehicles have served us well and hopefully will do so for a few more years to come.