[MikeDubreuil]

I think you may have overlooked this section in the readme:

Software Use:
Call Ping_Sonar() to update the distance reading. Call Get_Sonar_Distance() to return the distance between the sonar sensor and an object in inches.

The functions: Hardware_Interrupt_Sonar() and Timer_Interrupt_Sonar() are only meant to be called internally in the sonar software. You only need to call two functions: Ping_Sonar() updates the distance between the sonar sensor and an object and Get_Sonar_Distance which will return the distance in inches.

I'm not familiar with your application but give this a shot. Note that I added comments about adding a time delay. It may take up to 50mS for the sonar to find an object.
]

#include <p18f452.h>  // version 1.1
#include <usart.h>
#include "sonar.h"
/* Set configuration bits
* - set HS oscillator
* - disable watchdog timer
* - disable low voltage programming
*/

#pragma romdata CONFIG
_CONFIG_DECL(_CONFIG1H_DEFAULT & _OSC_HS_1H,\
 			_CONFIG2L_DEFAULT,\
 			_CONFIG2H_DEFAULT & _WDT_OFF_2H,\
 			_CONFIG3H_DEFAULT,\
 			_CONFIG4L_DEFAULT & _LVP_OFF_4L,\
 			_CONFIG5L_DEFAULT,\
 			_CONFIG5H_DEFAULT,\
 			_CONFIG6L_DEFAULT,\
 			_CONFIG6H_DEFAULT,\
 			_CONFIG7L_DEFAULT,\
 			_CONFIG7H_DEFAULT);
#pragma romdata

int i;
int distance;

void main(void)
{
  int i=0;

  Initialize_Sonar();

  OpenUSART (USART_TX_INT_OFF &
 		   USART_RX_INT_ON &
 		   USART_ASYNCH_MODE &
 		   USART_EIGHT_BIT &
 		   USART_CONT_RX &
 		   USART_BRGH_HIGH, 129);  // open USART with baud of 9600
  Ping_Sonar();
  /* You might want to consider adding a time delay here */
  while(i<10)
  {
	distance = Get_Sonar_Distance();
	Ping_Sonar();
	putrsUSART("A");
	putrsUSART(distance);
	putrsUSART("B");
 	i++;
	/* You might want to consider adding a time delay here */
  }
}