I can't see swapping 3.08 axles onto something. If you had 3.08s, broke one axle and had a spare, that would be a good fix, but otherwise, why bother? I have tried to give away 3.08 axles with no takers. Even with stock-sized tires, a 3.42 will work out better. I would want 3.73 for 31's. The theory was that 3.08s gave better mileage with 29" tires, but even on tires that small, the 700 can hunt in and out of overdrive with 3.08s and I'd be surprised if 3.42 gave any less economy in real life. Now if you didn't have overdrive, those tall gears would pay off, but a 0.71 OD is no small thing.
The nice thing about 91 axles is the 30-spline shafts. Some people want the axleshafts, carriers and spider gears over the older 28-spline ones, so even if you send the housing for scrap, you might be able to sell those parts or hold onto them as spares.