I suggest posting a thread in the forum jeep club of the Amazon, the Pessaol will help for sure! And have done several times the way "Manaus-Porto_Velho.
Heavy rains here in the Amazon are a climatic characteristics. Depending on the date / month of travel the road to dry and "easy" transit, other times like January / February it rains a lot and the road becomes a big rut.
Greetings Tássio
Following is the site of the Amazons jeep club:
http://www.amazonasjeepclube.com.br/forum/index.php