UPDATE: I have (I think) overcome! Even though the TFI tested good and I didn't find any visible issues with the coil, plugs, wires, etc, something was definitely affecting the spark.
Since I was at my wits end I did the only thing I had not yet done. I swapped out the distributor (and TFI). Low and behold, it worked! No more bucking under load, even with accessories running! Idle is great, and everything is good in the world (for now).
I appreciate all the helpful responses and suggestions. It definitely helped me narrow my focus and confirm a few things. I'm still not sure if it was the pip sensor or TFI, but whatever it was, hopefully it's fixed for good. Fingers crossed!
In the wise words of the Drive-by Truckers..."keep dropping the hammer and grinding the gears"!