The shower needs a 2" drain line, drain, and p-trap (well, the p-trap could be larger as could the drain, but most people don't want a bigger drain in their shower unless it is going to be a car wash). You can utilize the larger pipe after the p-trap. A toilet drain line does not have a trap (it's in the toilet). The shower should be vented as well. If you are going to tile this, check out
www.johnbridge.com for help with tiling and building the pan properly. The drain on a shower works best (in most cases) if it is centered, so you will likely want to move things to there while installing the required trap.
