USPS new module not working on our Website

I uploaded the latest USPS module and it is not working correctly on our site.
We are getting the following error on a checkout from our site on this page "/index.php?main_page=checkout_shipping", I have the module in "screen" mode to see errors on screen:

-2147219085Rate_Respond.;SOLServerRates.RateV4_RespondInvalid XML Element content is incomplete according to the DTD/Schema.
line= 0 pos= 17551000440

View Source:
USPS build: 2013-07-28 Server: production Quote Request Rate Type: Online USPS Options (weight, time): --none-- USPS Domestic Transit Time Calculation Mode: NEW Cart Weight: 1.8 Maximum: 50 Tare Rates: Small/Medium: 0:0.03 Large: 10:0 Handling method: Order Handling fee Domestic: $4.00 Handling fee Domestic: $5.00 Decimals: 2 Total Weight: 1.83 Pounds: 1 Ounces: 14 Length: 8.625 Width: 5.375 Height: 1.625 ZipOrigination: 19124 ZipDestination: 08054 City: Mount Laurel State: New Jersey Order Total: $35.00 RESPONSE FROM USPS: ================================== -2147219085Rate_Respond.;SOLServerRates.RateV4_Respond Invalid XML Element content is incomplete according to the DTD/Schema. line= 0 pos= 17551000440

Everything worked perfectly until I uploaded the new module, I had to because the old one stopped working.
I don't really know what it could be, json is not available on our version of PHP, but it looks like it can pull from CURL, which does work. could it be the new plugin working with the old version of PHP and Zen??

Specs:
PHP 5.1.6
Zen Cart 1.3.9h
FTP install
Dedicated server

Here are the PHP errors:

Notice: Undefined variable: current_page_base in /www/mysite/includes/modules/shipping/usps.php on line 146

Notice: Constant NAVBAR_TITLE_1 already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 11

Notice: Constant NAVBAR_TITLE_2 already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 12

Notice: Constant HEADING_TITLE already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 14

Notice: Constant TABLE_HEADING_SHIPPING_ADDRESS already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 16

Notice: Constant TITLE_SHIPPING_ADDRESS already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 17

Notice: Constant TITLE_PLEASE_SELECT already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 20

Notice: Constant TITLE_CONTINUE_CHECKOUT_PROCEDURE already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 26

Notice: Constant TEXT_CONTINUE_CHECKOUT_PROCEDURE already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 27

Notice: Constant SET_AS_PRIMARY already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 29

Notice: Constant NEW_ADDRESS_TITLE already defined in /www/mysite/includes/languages/english/checkout_shipping_address.php on line 30

Notice: Undefined index: US in /www/mysite/includes/modules/shipping/usps.php on line 727

-2147219085Rate_Respond.;SOLServerRates.RateV4_RespondInvalid XML Element content is incomplete according to the DTD/Schema.
line= 0 pos= 17551000440

View Source:
USPS build: 2013-07-28 Server: production Quote Request Rate Type: Online USPS Options (weight, time): --none-- USPS Domestic Transit Time Calculation Mode: NEW Cart Weight: 1.8 Maximum: 50 Tare Rates: Small/Medium: 0:0.03 Large: 10:0 Handling method: Order Handling fee Domestic: $4.00 Handling fee Domestic: $5.00 Decimals: 2 Total Weight: 1.83 Pounds: 1 Ounces: 14 Length: 8.625 Width: 5.375 Height: 1.625 ZipOrigination: 19124 ZipDestination: 08054 City: Mount Laurel State: New Jersey Order Total: $35.00 RESPONSE FROM USPS: ================================== -2147219085Rate_Respond.;SOLServerRates.RateV4_Respond Invalid XML Element content is incomplete according to the DTD/Schema. line= 0 pos= 17551000440

Fatal error: Call to undefined function json_decode() in /www/mysite/includes/modules/shipping/usps.php on line 1172

It looks like your PHP version is old if json_decode isn't found (this was introduced in PHP 5.2). You might need to change hosters if your hoster isn't staying up to date.