Rear 12 bolt until '82ish. Decent ring gear strength. Count the bolts on the diff cover and that will answer your question. The front will be a 10 bolt.
Lunchbox lockers for both, 33's or 35's, keep in mind that you are running 1/2 ton axles and go have fun. Especially don't let it get bouncing or spinning or hard throttle if the front tires are turned.
I successfully ran that same front axle for almost 10 years with a locker and 38's. I also successfully blew up that 12 bolt rear axle twice with a 396, 33's and a heavy right foot (loved to do burnouts AFTER I'd get it rolling and clutch fully engaged).
A good friend of mine stuck lockrights front and rear in his stock '87 blazer axles 15 years ago, along with 33's (later 35's) and takes it hunting and moderate Moabing.
Keep your eyes open for the 14bolt that Carl mentioned, along with 8 lug rotors and hubs for the front, and swap them in when you can. Get 16" rims and tires...that way you won't have to switch tires when you put in the rear axle.
