I've got some good news and bad news.
92 D250 Cummins 5.9L auto, at 240K miles in my driveway.
The good news is that it's probably not the speedometer itself. Early on the speedo kept dropping to zero, but a slap on the instrument panel made the speedo jump: loose wire! The cruise control still worked fine with this one.
There's more than one speed sensor on your D250. There's one near the fuel injection pump, and another speed sensor on the transmission. The abs sensor is on the differential, but if I remember right, it feeds the abs section of the control module, which controls the RABS valve.
The bad news is that I only know this because we've replaced or had issues with all of the above and more.
We've driven long distances - 1400 miles a few years ago, 900 miles last spring, because a sender went out and the overdrive quit. This has been accompanied by the cruise control going crazy. The speedometer worked fine when this was fixed.
Last week the fuel injection pump speed sender went out for the third time: the indication was the speedometer dropping to zero occasionally, or jittering, and reading 15-30 mph while the truck is at a standstill. It's doing it again, so it's back to the shop tomorrow because, although the sender was done, the problem is not. No cruise control again, jittering needle and 15-30 mph indication while stopped.
I'm suspecting a faulty ground for the engine electrical. Our year uses a 12V system with a ground reference. Both the transmission sender and the one on the fuel injection pump are shaft driven signal generators with a ground and a connection to the control module. You should be able to spin the shaft with a drill motor and get a voltage output. It's dead if you don't. I think there is also a big warning to use low impedance test equipment around the computer inputs...check that, very important, but I'm not sure about the terminology.
If the control module (we've replaced that too) doesn't get a signal for ten seconds or so, it stops sending a signal to the speedo, and you'll lose the speedometer. If you have the cruise control set, it will think the truck is not moving fast enough and floor the accelerator. Excitement! Danger! Just like a Toyota! Just turn off the cruise control.
If the A/C suddenly stops working, check the relays and fusible links on the left wheel well (from driver position) in the engine compartment. If those are good, it might be the module.
The abs sensor messes up the rear wheel anti-lock system, but didn't seem to affect much else. If that goes out, you're just drive a truck without rear abs. This was the only one that wasn't a sensor failure for us, even though the local dealer replaced it three times, and then tried to sell us a PCM. THAT one was just a broken WIRE!!
We had the automatic tranny rebuilt at 140K because the previous owners hauled a big 5th wheel and the clutches were burned up. Put it back in and it started falling out of overdrive. The tranny shop pulled it back out, cracked the case and couldn't find anything wrong. Put it back in. Still falling out of overdrive, they pulled the tranny again, repeating above, and it was still falling out of overdrive. We gave up, took it to another shop where they checked the speed sender.......
The Cummins keeps hummin, unless the fuel cutoff solenoid fails..... if it suddenly quits, check that first. We didn't, and took a 25 mile towtruck ride to find it.