I have a 1994 Ram 2500 4X4 pickup with automatic transmission.
Just left on a road trip today. I was on the interstate with the cruise control turned on, when suddenly the speedometer needle dropped to zero and the truck dropped out of cruise control. I was still able to use the fuel pedal. After a few seconds the speedo started working again.
I turned the cruise control on again and it behaved ok for a while, but ended up doing the same thing a little while later. So I turned off the cruise control and drove without it.
I just exited at a rest stop with wifi so I could post this.
This is the first time I've had any problem with the cruise control or speedometer.
Only a few minutes passed between turning off the cruise control and exiting for the rest stop; so I don't know yet whether keeping the cruise control turned off will make the speedo function normally.
I just wanted to post. Maybe somebody has had the same thing happen and know what the problem might be. I just don't know if the problem is caused by the cruise control or the speedo.
I'm going to be driving for several hours, and I'll get back online later and report what happens.

 

I'm guessing the speed sensor unit has died. It is mounted on the transfer case I believe. When mine quit I had to do some wire splicing because the new one is a little different than the old one. Not too tough though. Before you remove it be sure to note the clock angle the old one is mounted at. Different gears require different positions.

 

The speedometer works fine. It only has a problem when I turn on the cruise control.