'99 F350 O/D Light blinking

[dperone]

I picked up a late 99 F350 dually the other day, and I'm working through bugs. It's 2wd automatic. The original owner said he just had the torque converter replaced about a hundred miles ago. The truck has been sitting for a year or so while he was deployed. I had it yesterday, and the speedo went out and the truck wouldn't shift into overdrive and the O/D light started blinking. Shut the truck off, speedo came back and light stopped flashing. After I drove it about 3 miles, the light started blinking again and it would not shift into overdrive, but the speedo stayed working. Scanned it, came up with a P0500 code and 2 shift codes, 2-3 and 3-4.
Talked to our trans guy, he said it sounded like the speed sensor on the rear, so I replaced it. It had some gunk on it, but not too bad. After that, it ran fine the rest of the day and all day today, until late in the evening when the light started flashing and I lost O/D again. Shut it off, turned it back on, shifted fine for about 20 miles, then the light started flashing, but I still had overdrive, although it does feel like it's shifting into overdrive weird. I'm going to scan it in the morning, but I was wondering if anyone had any ideas. I'm thinnking it's a sensor as I have had similar problems before with other trucks, but the fact that he had a t/c put in i has me a little leary that that could be the problem.
Thanks, Dave