Figure on a rebuilt 12-bolt with a new Eaton posi to run about $1500, unless you have a line on something less expensive. Multi-leafs, $225. Hardware (if needed, U-bolts, shackles, bushings, mounting pads, shocks, brake parts, etc, $???). You'd easily spend from $1500 to two dimes for this job. It's up to you if it's worth it. If it were me, I'd probably rebuild your 10-bolt with a Eaton posi, and some 3.73's. Then order new replacement mono's...maybe some slapper bars would help the mono's and keep tires from bouncing around on blast off.
Let us know what you decided and why.