The 5 Claptraps that potentially award an SDU in PT2 are the ones in Sledge’s Safehouse, The Lost Cave, New Haven, Tetanus Warrens, and Crazy Earl’s Scrapyard. Having an overstuffed backpack does seem to help the odds of receiving an SDU, although it certainly doesn’t guarantee them. The easiest way to overstuff your backpack is to get the maximum number of items you can carry, and then complete an easy side mission that gives you some type of gear as a reward. Bam, overstuffed backpack. My routine for hitting 72 backpack slots is usually as follows:
- Recover all 10 vanilla SDUs in PT1 (42 slots)
- Recover the SDUs on Zombie Island and in the Robolution (48 slots)
- Find the Claptrap in Lockdown Palace, recover his repair kit, complete the mission to rescue Athena, scavenge enough gear to fill my backpack
- Save/quit back to T-Bone Junction with a full backpack, turn in the mission to Athena, and use the mission reward to overstuff my backpack
- Farm the Lockdown Palace Claptrap until I get his SDU (51 slots)
- Play PT2 normally (usually just story missions), accepting each Claptrap Repair mission and getting the repair kit (but not turning it in)
- Once I have all 5 repair kits, head out and recover the PT2 SDUs on Zombie Island and in the Robolution, using these SDUs to overstuff my backpack
- With an overstuffed backpack, run through and farm each of the vanilla PT2 Claptraps until they give up their goods
- Once all 5 vanilla SDUs have been recovered, use them all to jump from 51 slots to 72
As @VaultHunter101 said, once you accept the mission “Find Steele”, you can no longer get the PT2 SDUs. So definitely make sure you get 'em before heading to the end of the game.
As far as the pearls question, it has to do with the fact that level 69 gear only shows up in vendors (which can’t house pearlescent gear) and from loot midgets (who don’t have pearls as a part of their loot pool).