You get the 6 lug 9.5" 14 bolt from a "light duty" (6 lug) C2500, key there being -C- 2500, the axle is very close to the same width as your existing C1500's 10 bolt. It's the 4x4 axle that is wider by several inches, so skip it. Get the u-bolt saddle plates while you're there; the axle tubes are larger so your old ones won't work. Use new u-bolts though, always. And of course, the conversion joint, which the # is in numerous 14 bolt swap threads, don't have it handy.
For the front - 88-91 can have the standard duty brakes which would use a shorter stub axle on the spindle, but yours being an extended cab should have the heavy duty brake package already with the slightly longer stub axle. With this you just swap the 6 lug rotors from the donor truck right on. No need to change spindles. The rotors are the same already, just 6 lug instead of 5.
Richard