Okay here it goes my brother is in Iraq and i am tryin to finish his monte carlo for him myself. I keep getting the run around so i turn to you guys. Working on his 1981 monte carlo has 88000 miles on it. It has a 355 with headers x pipe and flowmasters. Had new 350 tranny in it with shift kit. It has the 2:73 single wheel peel in it i believe. I am wanting to get it to be posi trac with 3:73 it is his daliy driver when he is home and i think that will be okay for him even if he is running down the high way! But i need to know what do i need to do this i dont want to spend alot at all. I was told the guts out of a 82-87 s-10 would work? Can you please help me do this the cheapest way possible parts wise. Thank you for you help. oh and the spline i think is 26 i think whatever it came with. well please let me know  

Sorry but no cheap way. You could look for a rear from a Monte SS in the boneyard. Supply and demand makes that somewhat pricey but most do it yourselfers can do the swap. Having the internals changed in the existing rear requires some expensive parts as well as a skilled mechanic to set up the rear with the new parts.