First off, why is it cracking? It sounds like the subfloor may be moving enough to cause problems. If there are water leaks and you have rot in the joists than you may be replacing more than just the shower base, especially since you have particle board flooring. If the base has been installed correctly then the wall sheeting/tiles will be overlapping the lip. If this is the case you will most likely have to remove the wall sheeting. If it hasn't been done properly, well, you're probably going to need to rip it all out anyway.More information and pictures may help.
Mick