Have 2 sunny pick ups one has a sr20det conversion already but not happy with stock back axle and driveshaft, can i use back axle and shft from an s12?

I wont bolt straight in, but the diff from the S12 can be modified to fit. The S12 diff may also be wider, which you could correct by using wheels with less outwards offset.
The solid axle s12 differential should be an H190, which will be plenty strong enough for the conversion you are are planning.

I run the S12 H190 diff in my race car with SR20DET - works a treat.

You will need to change the pick up points for the lower trailing arms, but the top ones should be fine. I is much wider than the standard diff, so you will actually need wheels with a much larger offset. I run +24mm offset on mine, but I also have 50mm flares & 225 tyres. Even with +45mm offset I think you might struggle to get them under standard guards.

When you say Sunny pick ups, I assume you have a b120,b122 or b140 utility/bakkie, not a b310, which is the model known as the Sunny here in Australia. The S12 diff can be made to fit, but all the trailing arm mounts would need to be remove, & the standard Sunny(b120) mounts welded on. If you measure the widths, the S12 diff is about 100 mm/4 inches wider than the B310 diff,which is wider than the b120 by 50mm/2 inches. So the rear brakes would be well outside the rear wheel arches. You could narrow the S12 diff Housing & axles, because you've already got it & the diff & axles are strong enough. As any alternative you could get the H190 diff housing from a C120 Vanette that had the A15, they have a 4.1:1 steel centre, are leaf spring already,although they have the same size axles as the H165.
Also,where abouts are you from? Not being rude or anything, but it always helps to know what advice to give
you. You might not have the model I've recommended to you easily available, it might have a different name, & it all helps to give you information you can use. Please fill out your profile.Thanks.

Well bugger me... missed the fact that it was a pickup.... which probably has a leaf rear end... forget everything I babled on about previously :(

Cheers for info, from Ireland and its near impossible find
any parts to suit my b120's, government scrappage schemes saw alot of old cars crushed. Have 3 datsun 1200's bought from japan. One has sr20det conversion but needs better brakes, drivetrain, suspension. Other is stock b120 which i use for everyday driving but want to lower it and build the a12, and the other is a crashed 78 b120 which im keeping for parts, My knowledge of datsuns is pretty limted so will prob be on a fair bit asking stupid questions so be patient :) will prob go down the s12 route sounds like alot of work, but have an s12 to strip down but just want to know what parts i can use on the b120 before i put any work into it. Cheers for info