Answer: Yes, unfortunately, there is a problem inside the transmission that is causing this. 2004 seems to be an especially bad year for truck transmissions, as we've seen many problems with them. If you checked the fluid, it is most likely burnt smelling and dark colored which is the first sign of trouble. You may also have a trouble code or codes stored that relate to the transmission and what is going wrong. It's possible there is a slippage condition due to worn out internal parts which would set a trouble code. Not sure how many miles you have on the vehicle, or if you've ever had the transmission serviced before, but to correct a problem like this, the transmission will have to come out and be disassembled.