Userscript AJAX request gives JSON-RPC error -32700

This is extremely similar to issues, EG: Request to Random.org API with ZendFramework2.
However, I think my problem lies somewhere in my data request.

GM_xmlhttpRequest({
    method: "POST",
    url: "https://api.random.org/json-rpc/1/invoke",
    data: {
        "jsonrpc": "2.0",
        "method": "generateDecimalFractions",
        "params": "{\"apiKey\": \"d2319b89-8389-4d24-b1eb-4dbd80009153\",\"n\": 10,\"decimalPlaces\": 8,\"replacement\": true}",
        "id": 15324815
    },
    headers:{
        "Content-Type": "application/json-rpc"
    },
    onload: function(response) {
        console.log(response);
    }
});

The error I'm getting is:

finalUrl: "https: //api.random.org/json-rpc/1/invoke"
readyState: 4
response: "{"jsonrpc": "2.0", "error": {"code": -32700, "message": "Parse error", "data": null}, "id": null}"
responseHeaders: "Date: Wed, 01 Apr 2015 02: 34: 08 GMT?Server: Apache/2.2.22 (Debian)?X-Powered-By: PHP/5.4.39-0+deb7u2?Content-Type: application/json; charset=utf-8?Access-Control-Allow-Origin:
*?Connection: Keep-Alive?Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept?Content-Length: 87?Keep-Alive: timeout=15, max=200?"
responseText: "{"jsonrpc": "2.0", "error": {"code": -32700, "message": "Parse error", "data": null}, "id": null}"
responseXML: null
status: 200
statusText: "OK"

So I'm fairly certain the problem is in my JSON request. I'm hoping that someone with more experience with JSON-RPC can help point me in the right direction.

You need to properly JSON encode the data before sending it:

var encodedData = JSON.stringify ( {
    "jsonrpc": "2.0",
    "method": "generateDecimalFractions",
    "params": {apiKey: "d2319b89-8389-4d24-b1eb-4dbd80009153", n: 10, decimalPlaces: 8, replacement: true},
    "id": 15324815
} ); 

GM_xmlhttpRequest ( {
    method:     "POST",
    url:        "https://api.random.org/json-rpc/1/invoke",
    data:       encodedData,
    headers:    {
        "Content-Type": "application/json"
    },
    onload:     function (response) {
        console.log (response);
    }
} );