If I only replacing 2 rear brake lines, do I have to bleed all 4 corners or just the ones I replaced? Thanks

BUT !

You'll end up bleeding all four (In the correct sequence)
'Cause you'll not be able to keep the AIR out of the rest of the system.

It REALLY not that big a deal to do it correctly.

__________________
'84 300SD sold
124.128

Lately, I've been working on my cars alot. Just wanted to know if I can get lazy, lol.
Thanks.

The rest of the "system" ( front brakes ) will be unaffected by bleeding the rear. Many times the replacement line can be bled at the last connection in a pinch.

It is good practice to bleed all wheels not because of air, but to change fluid through the entire system as regular maintenance.