I've been reading threads on lobro joints and push in shafts and I'm getting all confused.

Here's my diff

Differential
Which the little tag tells me is a 3.38

These are the drive shafts I have (haven't picked up yet)


driveshaft
I'm not sure what car these have come from, but they have drum back plates with them, but no drums.

Now I know the 338 is open, which is what I want (fitting atb later). What I can't make head nor tail of is what shafts I need.

I'm going to be running rear disks, so what else do I need?

Those look like the right shafts. What's missing from the picture is the hubs. If that is what you mean when you say they come with drum backplates then to fit Sierra discs you will need discs, calipers and most importantly the caliper carriers. You can then get an adapter plate which allows you to fit the calipers to drum hubs. If you aren't getting the hubs then you will need to source those as well.

Steve