you can try and use a ratchet strap or comealong and put pressure on the end of the axle (where the wheel would go) and then pound on it while tightening it up. If that doesn't work, then get out the cutting torch and start heating up the c-clamp and see if you can melt it out without damaging the axle shaft, as you have found those axle shafts are a really hard material.
1998 Silverado Z-71 Ext Cab. 5.7L Descreened MAF, K&N direct replacment, 32x11.5 R15 AT's,Posi rear end, True duel w/Magnaflows, Sub woofer setup, Remote starter.