The starter seems to engage the flywheel because I can see the fan turn slightly. Then if I keep the key turned to start it continues to engage and disengage and all the dash lights flash on and off. Nothing at all happens if the clutch is not depressed so I'm assuming that switch is good. Batteries are good and no corrosion on the cables.
Any thoughts are welcome.

 

Starter contacts may be bad a common issue, you do NOT want to replace the stock starter with a rebuilt unit from a parts store. The OE starter is MUCH stronger than the junk you will get elsewhere.

Look into replacing the contacts a very easy job and you can get good ones here Larryb - Dodge Starter Parts - Dodge Cummins Diesel

 

Load test your batteries, to make sure you don't have a dead cell.