It sounds like a classic case of band/clutch slippage, which is the good news because it is fairly easy to fix. The not-so-good news is that the replace the bands and clutches you have to have the transmission torn down all the way, which is time-consuming and costly.
Now, you can have the clutches and bands replaced — ready for the other shoe — but if you do, you may find the gears are also worn to the point where they won’t engage correctly and even if they try to engage, the gears will likely slip. The reason is that they have been working with weaker parts. As soon as you replace the weak parts with normal parts, the other parts that have worked with them usually go quite quickly.
Actually, this could go on down the light, through the input shaft and torque converter to the seals and output shaft and to the solenoids and valve body (the check valves may become very lazy).
Here’s what I think you will have to do: replace the transmission with a rebuilt unit. At once, you will solve your slippage problem and it will keep your vehicle on the road for another 100,000 miles or more with proper care.
Yes, it will cost about $3,200 to install the rebuilt, however, think of it this way. You can either spend the money on the transmission now or you can turn the truck in on a 2015 or 16 model and you will find yourself saddled with either lease or loan payments from three to five years. The payments will likely be over $350 per month, depending on your credit.
And, by the time you are finished paying things off, you may be facing transmission problems so you will be back in the same boat.
If you install the rebuilt tranny now you will keep your car payments to $0, which is likely where they are now, and all you will have invested is the $3,200. It’s quite a difference, isn’t it? How much of a difference would it be? How about $37,000 or more, depending on the equipment in a new pickup. Let me know the outcome, if you would.