As Jumpu mentioned not everyone who does a rotation at a particular institution will end up matching there. So my recommendations would be first to secure a PM&R rotation at your home institution if you have one. The last thing you want to happen is for your home institution to not rank you at all because they have not heard of you since you did not rotate there. Then choose the best place you would like to rotate at to get the best letter of rec from someone known in the field of PM&R. As Jumpu mentioned, I believe that the letter of rec is the most key piece of the pie, after board scores and med school GPA. The best shining letters of rec from someone well known can take someone who is a middle of the road candidate and put them in everyone's short list of must have applicants. By the way, you should probably have at least 2 PM&R letters of rec when you apply to give you the best chances.
Just because your institutions only allow you 2 official rotations does not mean you cannot gain experiences through other means such as research projects, shadowing experiences, using your vacation time to rotate at other institutions, etc. If you can't find out about any of these experiences, then you truly are not trying hard enough. One student I interviewed this past applicant pool had gotten a phenomenal letter of rec by a well known PM&R attending by doing a shadowing experience one half day a week for 2 years... Another resident I knew when I was applying for residency had told me when he was a med student he was doing research with a well known PM&R attending across the country as he was on the east coast while the attending was on the west coast. If there is a will for something, there is a way to get it done.