I have owned two sets of 255/80-17 Coopers in E, and they do chunk when exposed to our dagger-like rocks.
How is the service history for your truck? Since I got mine 2.5 years ago, i have done the following:
water pump/belts
thermostat
radiator and hoses
secondary air injection system (bypass waiting to install)
CVs (just rebooted mine)
alternator
brakes
all fluids, including radiator
headlight polishing
Sounds like you could use the Firestone airbags in the rear to deal with excessive weight. Also consider the Eibach kits, as they come with the coil conversion kit and many members on GXOR seem to like them.
seeing Homer's measurements, i just measured mine. 22" and 22.5" on the front, with 5100s and 'leveled;' and 21.25 and 22" on the rear with SSO 3/4" installed. I forgot to measure when I was stock, so i don't have a frame of reference. I fit 255/70 ST Maxx with fender liner forward and a good amount of front bumper corner cutting.