My text box in Prism is set to be full-justified, but prints as left-justified.

Prism occasionally has trouble with full justification, in some circumstances, or with some printers. Try making your text box in Microsoft Word (which--not surprisingly--is a better word processor than Prism). Copy and paste that text box into Prism. Now it will be a Word object in Prism. You'll get all the advantages of Word (tables, bullets, etc). An alternative is to use a program like SmartDraw and either paste Prism graphs into SmartDraw, or SmartDrawings (including text, and much more) into Prism.