Perhaps the 'stuff' isn't glue. To test if it's wallpaper glue, mix a NEW solution of DIF, and get yourself one of those 'fat' sponges that you use to wash your car. Lay some newspaper on TOP of a sheet of plastic along the base of the wall. (If you only lay newspaper down w/o the plastic, the 'ink' may stain what' underneath it.)
Try and wash the walls with the HOT DIF mix. If that's not working, let it dry. Then, take some medium sandpaper to it. See if that does anything for you. IF it does, go at it. Just be sure to do a complete and even job. Other than that, perhaps a Retail Wallpaper store can help.