Whatever you do, don't spend money to fix that 10 bolt if you plan to keep the truck!
Unless you really want it original, change the rear axle to a 12 bolt or 9.5" 14 bolt.
The 12 bolt came in 1/2 ton trucks up to about 1980 or 81. It has an 8.875" ring gear compared to the 10 bolt's 8.5, but the carrier, housing, and shafts are heavier. An axle from any 4wd 1/2 ton truck, blazer, or suburban from 1973 to 1981 will bolt right in, same spring pads, axle width, 6-bolt pattern, U-joint, etc. Just need 3.07 gears, which are pretty common. You get slightly bigger brakes too, which is nice!
A 6-lug 9.5" ring gear 14 bolt came in the rear of 88 and up light 3/4 ton trucks (7200lb GVW) as well as some 1/2 tons with the HD rear axle option. It's much much heavier, and from those vintage trucks will widen the rear track to better match the front on your blazer. You will have to weld on new spring perches though, and probably shock mounts. You'll need a conversion U-joint too, but they're cheap. I think about the same size brakes as the 12 bolt.
Stop by a nice mom and pop dirty auto repair joint and ask about junkyards or recyclers that deal in older cars... They usually don't advertise much, but the price is right if you can find them, particularly true of recyclers. Most times they'll even torch the U-bolts and shocks for you so you can just drag it home and bolt it in. I'd pull the cover and have a look as soon as you get home. If it's bad, take it back and exchange for a different one!
Good luck!
Chris