Certainly, the solenoid would be an easy thing to check by substitution. I would just swap it with one of its neighbours with a black cap (labelled A or B or C in the photo) and see if the problem moves with it.
You could also check the integrity of the solenoid wiring by measuring the resistance across pins 7 and 21 at the ECU plug and checking that you get around 6 Ohms.
If you let me know the serial number of your transmission, I can tell you its date of manufacture.
Phil