You just need to spray them with a good penetrating spray. I took mine off and didn't have much problem using a 1/2" drive breaker bar with the correct size torx socket.
The threads get corroded from all the road grime that is always getting sprayed up under them.
I lifted my bed up to do the Harpoon mod to my truck:
2005 F-250 CC/SB 4x4 medium wedgewood blue
5" lift, 5" Donahoe rear springs, 35x13.50 Extreme A/T's, 18x10 Ultra Predator wheels, 4.30 gears, Banks 4" exhaust, Airraid intake, SCT from Innovative, ARP headstuds, Bulletproof ficm, Scangauge II, Randy Ellis Design lightbar, Street Scene flares, Harpoon mod, Zoodad mod, LED 2010 taillights, dual color LED front turn signals, Pioneer avic-D3 with ipod and backup cams