new to me 84 F250 supercab 2wd 6.9 idi C6 auto cruise and air.

apparently i am not allowed to post pictures yet.

at any rate the cruise turns on but it surges. wondering if there is a common remidy for that?

 

I believe on the diesels there is a vacuum pump as a substitute for a gas engine's crankcase vacuum. The pump assists in setting the cruise like the gas engine vacuum, so I believe the pump is your problem. Other symptoms would be soft brakes (vac brake booster)

 

I think your diagnosis is correct, but when you have low vacuum the brake pedal is rock hard.

Also check to make sure the linkage is not binding and moves freely.

And keep an eye on your speedo needle. I believe your year uses a sending unit in the speedo cable, so if the cable is dry and the needle is jumping, that could cause a problem with the speed signal.