Have you disassembled the transmission control and cleaned, lubed and adjusted it? Have you replaced the control cables? Have you taken the cables off the transmission and tried it directly? This last one would at least tell you whether the problem is in the transmission or the control/cables.
I did clean and lube the control, however I have not changed the cable. I will first try your idea of removing the cable to test if it's the transmission or cable. Then I will see if I need to replace the cable. If it's the transmission, I'm guessing I might be out of my league for fixing it. Or is there a simple solution to loosening it?