You sound (understandably) emotional. You need to get over this really quickly.
Evidence and cool thinking are your friend here. You need as much evidence as you can that you are either meeting your objectives, or that your objectives are not specific / measurable.
It is largely irrelevant what you think your colleagues do / don't do as their objectives will be different to yours.
A PIP should have clearly listed why you are deficient, and what you need to do in order to resolve. Make sure you understand what those criteria are. In forensic detail. And make sure it is achievable. All documented and agreed. If this all happens verbally, make sure you take notes and then email it to whoever was present as your notes from the meeting.
I have used the PIP process to buck up the ideas of employees previously. It isn't always a means to manage people out, but I concede it often can be.
You also need to put aside what you think your objectives should be, or what you think you should be doing. The only thing that matters is whether you are delivering what you have been asked to deliver whether you agree with it or not. I only say this as many of the employees I've had on a PIP delivered some fine work, but it wasn't what they had been asked for, or what was needed. They were most disgruntled.