Forum:PT 2(.5) SDU's

I've read what it says on the mission page, and I managed to get the SDU from Sledge's Safehouse on the 5th or 6th try. However, I'm 0/12ish in the Lost Cave, 0/30+ in New Haven, and 0/7-8 in Tetanus Warrens. Bad luck? Not enough patience? Naturally, my inventory is full (48/48, after missing the one in Knoxx PT1) every time I try, though I've never pushed it past 100% full - does that make a notable difference, or is it negligible?

I've also heard that the odds are better if you pick them up "in order". A) Is this true? B) If it is, what's the proper order?

Anyway, is there any insight on the odds of getting these to drop? All it says on the wiki page is that the odds are better the more bloated your inventory is.