98 Silverado wont decelerate

I just bought this truck and it seems when I let off of the accelerator that it keeps going. When Im running about 60mph and let off the gas it keeps going for about 5-10 seconds then it feels like something releases and it starts slowing down. Is this a T/C - solenoid issue? I can hit the brake and it will lock or unlock whichever it does when the brake is applied but I tried it at 70mph today and I didnt hear it change should it still change at high speeds? I had it checked for codes and there werent any codes. How do you determine if a T/C solenoid is bad?
Thanks