Can you hook up the PICs directly to the 232 side of the 232 to 485 converters? At least this will prove it is a problem with the TI part. It would bite if you got the Max parts and found out it was something else! I replaced every part on an RF comm board (including connectors!), to find out I mixed up the RTS and CTS lines in my code. That was five hours better spent elsewhere. "The length of time debugging is directly proportional to the stupidity of the error." This I have gleaned from personal experience.
Ron