This probably isn't the best solution but I found a bit of a work around, I'll post it here just in-case anyone has anything to say or stumbles across this in the future
Basically the idea was that I created some new fields for the client itself, gave them some relatively unique names, and then retrieved them later in the serverCmd function
function fxDTSBrick::setVehicle(%obj, %data, %client)
{
%client.RPGSetVehicleObj = %obj;
%client.RPGSetVehicleData = %data;
clientCmdMessageBoxYesNo(%title, %message, %yesServerCmd);
}
function serverCmdRPGSetVehicle(%client)
{
%obj = %client.RPGSetVehicleObj;
%data = %client.RPGSetVehicleData;
//code that uses %obj and %data here
}