Without knowing the definition of INT32U
it's hard to say, but you probably want:
printf("Task Priority %d - Used: %u; Free: %u\n",
prio, stk_data.OSFree, stk_data.OSUsed);
or:
printf("Task Priority %d - Used: %lu; Free: %lu\n",
prio, stk_data.OSFree, stk_data.OSUsed);