Check your driveline play at the pinions. It's normal to have some, and it'll clunk. You can test this by speeding up letting off the gas then hitting it again on an uphill. You'll get some clunk on any LR, if it's a whole lot you may need some toe adjustment in the diff, but that's a major pain. Could be on either end of either shaft though.

 

Was there any resolution to this problem? It seems I have the same thing happening.

It's far less pronounced at speeds below 55 mph, but once above 60 mph, I get an oscillating surge in the throttle (the resulting acceleration/deceleration can be felt and is rather annoying) which results in a 1-2 mph fluctuation at speed.

I was thinking I may have a vacuum line leak, but I decided to check here to see if there were any other possible causes for this.

Was there really a service bulletin for the ECU to be reprogrammed to solve this problem?

 

Because of driveline slack some "slop" is normal, the cruise will not accomidate this because it doesnt know about it.
When the cruise gives it gas and the truck doesnt respond then it will give it more gas, then the driveline slack is taken up and the truck surges forward, now you are going to fast, so the cruise lets off the gas, then the slack goes the other way, now to slow, so back on the gas again, and so it repeats like that.
The cruise on my DI is kinda like that way at low speeds (25-45mph) on hilly terrain, but at highway speeds it is as smooth as silk.
You can help with the slack buy putting your tires 35psi front and 45psi rear as well as replacing your differential and transfer case fluids with 85w-140 gear lube.
The thicker lube will take up the slack and the driveline will be quieter.
Also running your transmission in 3rd gear while under 45 will help.

 

mike its normal, the cruise control on disco 2's suck hard core. i cant belive anyone really bothers to use them. otherwise if it dosent bother you it goes away after about 5 miles. I think mike is over reacting to your post. theres nothing wroung with the truck, the cruise control just sucks.