Seems to me like it is not so much a matter of riding the brakes as it is a matter of forcing the clutch packs to slip while all up against one another, thus generated a LOT of heat. That is what is causing the rears to go out, in my opinion. I have had several one legged rear cars that I used the brake method for doing burn-outs. One car was even a 13 second car on slicks. I never had any problems with that car and it was a totally stock c-clip Chevy 8.2" 10-bolt doing this for two full seasons.
It's the darn heat generated by forcing the two tires to spin at different speeds as caused by unequal brake pressure. Sure, the tires probably spin at almost the same speed the first few times you do it. Then, as the clutch packs get more and more glazed and worn out, soon the tires are spinning at way different speeds, and that is why you notice it being so obviously uneven the time or two before the final death blow. Those last few times, the clutch packs, gear oil, axles, spider gears, are all getting soooooo hot that something has to give.
The solution? Install a linelock for under $100,(I need to as well), or just drop the clutch at a reasonably low RPM to get the tires started spinning. Sure, the truck will quickly walk forward as mine does, but that's better than burning up rear end parts. Hope my $.02 helps.
Greg
