*Rocket Spanish* is the best program I know for someone just beginning to study Spanish. To be honest, though, I think you probably need something more advanced. Of course, Rocket Spanish does include an _excellent _Advanced book (in pdf format) that offers very clear and useful discussion of the preterite and the imperfect, as well as prepositions (a, en, de, hacia, con, desde, entre, sin, hasta), uses of por and para, pedir vs. preguntar, and lots of other topics. But the audio portion of the course doesn't deal with these topics; it's much more basic, and you sound as if you're far beyond the audio portion's level. Rather than spend all that money just to get the Advanced book, I'd suggest you either get a different program or forget about a program and just get some helpful books.
I've had some experience not just with *Rocket Spanish *but also with *Learning Spanish Like Crazy*, *Pimsleur *(levels 2, 3, and Plus), and *Rosetta Stone* (just a bit--I didn't like Rosetta Stone and so didn't continue). Of these, I think the one you'd find most helpful is Learning Spanish Like Crazy. The problem is that it teaches by example--it offers only minimal _explanation_ of when to use the imperfect vs. the preterite, for example, though it devotes several audio lessons to these tenses. If you think you need or want this audio portion, I think you'll get it better on Learning Spanish Like Crazy than on any of the other programs I've mentioned.
After two months in Latin America, however, you may not need the audio as much as you need something to help explain Spanish grammar and usage. For this, I'd highly recommend the three books in the *Practice Makes Perfect* series: *Spanish Verb Tenses *by Dorothoy Devney Richmond, * Spanish Pronouns and Prepositions* by Dorothy Richmond, and *Complete Spanish Grammar* by Gilda Nissenberg. All three are available in paperback for reasonable prices (I think I bought them all at Amazon.com). There's a certain amount of overlap, though each book also covers material none of the others does. They offer good explanations and lots of exercises that will probably help reinforce and extend your vocabulary as well as teaching you grammar. *If you want to try just one, try Practice Makes Perfect: Spanish Verb Tenses.* It covers not just issues like ser and estar, the personal a, saber and conocer, stem-changing verbs, reflexive verbs, gustar, etc., but also the preterite, imperfect, future, conditional, present perfect, past perfect, future perfect, and conditional perfect, as well as the subjunctive, the imperative, and the use of the passive voice.
I hope this helps.