There are a number of level shifter ICs available.
However, you need to think of what is required first: by the way you phrase your question I understand that you want to output a signal from the PIC and input it into the ZigBee. If that is all you need to do then you might get away with a couple of resistors as a voltage divider.
If you want to input a signal from the ZigBee to the PIC, then you can try to connect them directly - I don't recommend this but the 3.3V "high" voltage can be above the threshold to trigger the 5V PIC to recognise the signal. However the better way is to use a transistor .
If you want a bi-directional signal, then there are bi-directional level shifter IC's (try a Google search as there are many manufacturers, chip footprints and sizes etc.).
As for the slew rate, that really depends on the situation - it must allow the signal to pass as fast as you need it to. For example, if you are connecting 2 UARTs at 9600 BAUD, then the slew rate can be a lot slower than of you want to use 921,600 BAUD; however the timing inherent in an asynchronous protocol can get over the 'jitter' that be seen on the input side. If that is important, then you might be able to use a Schmidt trigger input to overcome it.
Susan