The parts stores are correct you will need the new manual hubs and the conversion kit. The conversion kit consist of the correct hub nuts for the manual hubs (the auto hub nuts are totally different and wont work with manual hubs).
Here is the parts you will need:
(I have been running the Mile Marker brand for 2 years now with no problems)
$300 is far fetched for the parts.
Swapping manual hubs is super easy to do. Pull the old automatic hub off, remove the nuts, put new nuts on, install new hub. Remember, it takes a special socket to get the auto hub nuts off and another special tool to put the manual hub nuts on.
Most auto parts stores (ie: O'Reilly Auto Parts, AutoZone, etc.) have these sockets. They run about $10 each. While you are at the auto parts store, grab yourself a repair manual. They are pretty detailed about removing and installing the lock outs...
If you need anymore help with them, shoot me a PM...