Aaargh! Recurring code 22-16

[JohnParker]

I have an occasional, recurring code 22-16 on my transmission at start-up that locks out any shifting. When I go through the reset procedure and restart the engine, the "Do Not Shift" light on the dash goes off and I can drive nornmally. The next time I start the engine, the light may or may not come on; but if it does the code will clear and the unit drives and shifts normally.

The code, as I understand it, indicates a malfunction in the output speed sensor.

Questions:
Is this something that requires immediate attention?
Is this something that can be fixed by a semi-competent do-it-yourselfer?
Is a replacement sensor available over-the counter?
Is this something that requires a tow to an Allison shop?

Any guidance anyone can provide to a newbie who is trying to save a few bucks will be most appreciated.