How do postdoc proposal and PhD proposal differ?
It seems obvious that while some forward-looking ideas may have been included in the thesis and defense, a proposal to develop them should have much more than just a concept -- timelines, deliverable milestones, cost of equipment (for physical sciences and engineering) or travel or payments for volunteer research subjects, what approach to publication will be used, etc.  In short, mostly the same things that go into a grant proposal.

On the other hand, the methods of the thesis are not so important and should be condensed to an executive summary.  The only part that should really be common is the proof that an important problems is addressed -- if the resulting book will have no impact, there is no point funding the effort to finish it.



Disclaimer: I'm neither a social scientist nor in a postdoc.  But I do know what my employer would ask me to prepare in a proposal for funding for elaborating my dissertation.