Kens04Silverado

My son used my truck to go to work Monday night, and I got a call from him Tuesday morning when he got ready to come home. My truck ('04 1500 6.0 Automatic Trans) would not shift out of park. I replaced the cable 3 or 4 months ago, so I didn't suspect it to be bad. After reading other info on this particular truck model, I replaced the shift interlock solenoid, which didn't solve the problem. (I now have a spare solenoid). We finally got it out of park; but, it's like it is stuck in park. Unfortunately, the tow truck driver had to drag the truck up on the bed of his truck to get mine to my house. He advised me to check the cable again, check any fuses related to the ignition and transmission, and the lever mounted on the transmission that the shift cable connects to (it will move when the shifter is moved; but, does not go in gear). Can anyone advise me of 1. what fuses to check, 2. should I replace the shift cable again, 3. is there anything else I should check before considering a transmission shop? I can do some work on it; however, I do not know how to work on the transmission itself. Any suggestions on things I can check before considering a transmission shop will be greatly appreciated. Note: The truck has 450K miles on it, and has yet to give any actual engine or transmission problems, and hopefully it's something minor now as it runs strong and shifting prior to this has been no problem.