Presumabley the LCD display works OK if driven from a PC ?

Normally a backpack LCD would expect to receive RS232 levels. Your seting of "84" is for 9600 baud TRUE. Unless you are using a MAX232 to perform the translation from TTL to RS232 I believe you will need to use INVERTED. Try changing the "84" to "16468" as shown in Appendix A of the PBP manual.

You also dont say what speed your PIC is running at and the manual includes the following warning "9600 baud may be unreliable with 4MHz clock"