Hi,
* There is no internal 37kHz mode on the 628A. It's either 4MHz or 48kHz.
* You can RUN the PIC at 48kHz.
* You can NOT make PBP aware of it - ie. no DEFINE OSC for such low frequency.

Why do you NEED to run it at such slow speed? Power?

If so then one or more of these ideas might be worth to consider:
* There are modern parts with lower current consumption.
* Use the switchable oscillator and run it at 4MHz when needed, switch it back to 48kHz when you don't.
* Use the sleep feature and wake the PIC up when needed.

/Henrik.