To see if the spool valve is under the transbrake solenoid, you do not have to remove the pan. The solenoid will unscrew and you should see the valve. I use a magnet to pull it out if it won't spring out. If the valve isn't there, then you will have to pull the pan to see exactly which transbrake valve body you have, so that you can replace the valve (not interchangeable between brands) or the complete valve body.
Since your solenoid will go in (reverse should be working even if there isn't a spring to make it come back out and release reverse) and you don't have reverse it makes me think that the spool valve was left out or is not the correct one for the valve body.
Bill Mitchell