This morning I had overdrive and a working speedometer; fired it up
to head home this afternoon and I don't. Any ideas?

1995 2wd 200,000 miles.

 

check the speed sensor, located on the top of the rear diff

 

I didn't know there was a speed sensor on the rear- I did fiddle with the one on the trans where the speedo cable used to go. Went so far as to chuck the square end in a drill- didn't expect any results and didn't get any.
I took it to the local Cummins guru across the river in Ravenswood, WV- guy by the name of Brian Vannest. It was overdue for a trans service anyway and he's putting synthetic in both trans and rear.

Since my truck was out, I pulled a trailer 140 miles north of here with a 3/4 ton chevy with a 350 gas motor and auto trans, 3:73 4wd, and hauled back 3 1/2 tons of chicken feed- up empty and back full I averaged 8.3 mpg.
I get 14 on the same trip with my dodge- makes plugging in a block heater seem like a very minor inconvenience.

Thanks for the help.