They most definitely have GE enrollment centers at DEN, ORD, and IAH. But you can't just drop by an interview office and get immediately enrolled. The best way to do it is to enroll here: www.globalentry.gov. You fill out the online app, pay the nonrefundable $100 fee (for a 5 year term)..and in a few days you'll get an email with "conditional approval"..assuming you meet the criteria. You will then be able to schedule an interview online at an airport location. The GE enrollment center is outside security at most locations, but in the locations where it's airside you can use your appointment letter to get past the TSA.

The interview takes at most 15 minutes and they will take your photo and fingerprints.. you're then instantly approved and you can enter your PASSID number (which you can view at your online profile and on the GE card they will mail you) in your United profile and on each of your reservations.. this will get you PreCheck every time (or, virtually every time.. I'm 100% myself since getting GE).

mahasamatman mentioned Nexus (for expedited Canadian immigration) at half the price which also includes GE..I have this myself and this is true..however you must interview either at a US/Canadian border location or in select offices in Canadian cities..or at Boeing Field in Seattle (and the Nexus interview wait times at the Seattle location are significantly longer than for GE). Probably not practical for someone in Texas.