I'm trying to lower my shield/abs evenly to those levels
Don't do this. More mitigation stats are always better. If you're lowering your shield/absorb arbitrarily without directly increasing your Defense to compensate, you're operating counter to the intended purpose of the stat budget: more is always better, even if it skews the optimum ratios a little. You want to maximize your budget while getting as close to those ratios as you can.
How can I swap shield for defense? Is there a workaround I'm not seeing?
You can't and that's one of the reasons why I don't feel the need to tell people to follow these numbers explicitly. Defense rating doesn't compete with Shield/Absorb rating on anything but augs. No matter what, the Shield rating for *every* tank class, assuming maximum mitigation ratings, barring the PvP passive relics, will have 600 Shield rating. Shield rating is, functionally, a value with a static minimum based on specific tiers of gear. These calculations don't really factor that in (not that they couldn't, but it would make the math way more complicated).
Like I've been saying all along, don't work too hard on following the values *explicitly*. They're a guideline for what the optimum ratios are at various stat budgets, but they don't account for the numerous other factors, like situational effects and the complexities of sectionalized itemizations (since Shield/Absorb/Defense rating come in discrete chunks at a time rather than in perfectly fluid quantities and aren't universally interchangeable in every way).