I'd be willing to bet it's been sitting so long that the grease has hardened and is causing the pawls to stick. This is common with whatever Shimano uses for grease.

I'd remove the freewheel (instructions here: http://www.parktool.com/blog/repair-...ewheel-removal ), remove the seal on the back of the freewheel and thoroughly soak it in kerosene. After a good soaking, spin it to get the pawls working again. Then soak it again to wash out any particles of hardened grease. Lather, rinse, repeat until you're happy with the action. Then run some heavy oil into the bearings from the back side to keep it working. Snap the seal back on, reinstall the freewheel, go ride.