I've got 2x3750 switches with 4 physical links in between. I've bundled all the physical links using PAgP, but I want ONLY 2 links to be active at any single time. How do I go about this?
That's not possible with PAgP. Traffic is always aggregated across the whole LAG trunk.
Splitting the trunk in two groups of two ports plus spanning tree would work (switching over to the spare trunk when the default trunk's bandwidth decreases due to link loss), but when one physical link in each group fails you end up with two single links and one of them blocked by STP.
LACP could - in theory - support this scenario by limiting the number of active links. While possible in the protocol, I don't think any hardware actually supports that. Usually, the port number limit is hardcoded on a device.
If you think about it there's extremely little point in trying to limit the number of active trunk members.