The factory figured 23 mph because lugging the drivetrain is bad news for the freewheeling clutch rollers and cage. These pieces can't take much in the way of side loads, and they're difficult to locate if they need replacement. The planetary gearset also suffers from this stress. It's a good idea to wire an idiot light as a reminder that the solenoid is engaged, It's all too easy to forget and "bog" down.
The 'Burb page diagram is interesting but overly complex if you just want to control the solenoid. All that "split shifting" may be fun for a while, but a bother to do all the time. All you need is a toggle (and light) between the power source and solenoid.
