I'm including here a sketch which you sent me during our e-mail discussion, which may help others visualize the phenomenon.
I confess that despite your e-mail answering some of my requests for more detail, I'm having trouble visualizing exactly what you saw. In particular, I can't tell whether the light took the form of moving "sunbeams" traveling through clear sky near the cloud, or whether you saw moving spots of illumination on the cloud itself. Your description seems to suggest the latter.
Based on your detailed description of the light over e-mail (as bright as sunlight passing through the cloud, long-lasting, and colorless) I don't think it could be caused by lightning, or that it could be a "halo" effect caused by refraction through ice crystals ... despite the fact that the sun and cloud were at the correct angle (22 degrees) for halo effects to occur.
Instead, my best guess is that there were small clouds or other objects passing between the Sun and the thundercloud, and that these clouds cast their shadows upon the back of the thundercloud. You saw the shadows through the translucent thundercloud. As the little clouds were carried by the wind, their shadows moved, causing an apparent motion of the light shining on the cloud.
I may be completely misunderstanding what you saw, but this is my best guess based on my limited visualization of your observations.
A very good book discussing strange sights in the sky and other unusual natural occurrences is "The Handbook of Unusual Natural Phenomena", by William Corliss.
Keep looking up!
Try the links in the MadSci Library for more information on Earth Sciences.
