I make the mainline to UX transition through ORD pretty frequently (often flying SBN-ORD-LGA and vv) and can tell you that provided everything is running smoothly, you have plenty of time, even if you are walking from F to C or C to F. When things begin to back up at ORD you can run into problems, especially given that the UX folks like to cancel the flights that they see as within driving distance.

There is also bus service between ORD and MKE that runs fairly frequently. IIRC it is operated by Coach USA. My preferred backup would be a rental but a one-way can sometimes be pretty pricey.