Tested batteries and terminals and wont get a crank. 2000 stock dually, 5 speed.

Try jumping across the starter and see if it engages. If not I would start with the contacts in the starter could be bad.

It could also be the clutch switch in the cab. 

Ya if you have a test light see if you are getting power down to starter. Could be fuse, relay, clutch switch, wiring in the steering column, starter.

Double check the starter solenoid fuse I just had mine crack just a week ago.

http://forum.mopar1973man.com/index.php?/topic/8638-failed-to-start/?hl=start