Toyota Repair: 98 Camry LE 2.2L 4-Cyl Will Not Shift to OD, temp gauge, d light

Question
My Camry stopped shifting into OD, and I've been told I need to replace the transmission control solenoid and that it costs $100 - $250, but is a quick fix because it's on the outside of the transmission and easily accessible.  Otherwise, the transmission seems to shift OK.  Please advise.  Is it likely the solenoid?  What would be the part number?  Where would you buy it?  Is it easy for a beginner to change.  Thanks, Dan

Answer
It could be the solenoid but I just never replaced any of them for this problem, does the O/D light flash? Does the engine warm up to at least 1/2 on the temp gauge, the o/d won't work if the engine doesn't warm up, if the engine stays too cold try replacing the thermostat first.