My daughter describes this problem with her 01 Cherokee. Any thoughts?

After driving in heavy rain and sitting for several hours in rain, she is unable to engage the transmission. The vehicle starts fine, but she claims she then cannot depress the brake pedal. It doesn't budge. She can, however, move the gear selector into Reverse. The transmission doesn't engage. She then turns off the engine, restarts, everything is fine. This has happened twice.

Has anyone seen this before? Any ideas where to start? Thanks