Does the 2500 have a mode button? On the 3000, if that is pressed it delays the downshift (kind of like Eco mode) to prevent constant downshifting on gentle hills. The display should say Mode On or something like that. It's possible that the ECM has been programmed to turn that on with the cruise control.
Jerry & Jeanne
1996 Safari Sahara 3530 - 'White Tiger'
CAT 3126/Allison 6 speed/Magnum Chassis
2014 Equinox AWD / Blue Ox