-- how many miles are on that GMC of yours? You said you've put 12k on it yourself, but its not clear immediately if you're the original owner (and if not, what the total mileage on that van is.) Wondering how worn the various tie rods/ball joints/bushings might be in your van, and if that isn't the logical culprit if the van has higher-mileage on it.
A lot of worn-out joints can actually act totally fine most of the time while the vehicle is loaded up and driving along on smooth pavement.....but when a sudden jolt to the suspension happens (especially if the suspension is side-loaded somewhat, on a long curving stretch of freeway), the joint can swivel unpredictably and create all sorts of oscillating suspension movement (and radically changing/swinging steering geometry). I had an idler arm that was going bad in an old Dodge sedan, and it wasn't totally obvious until I hit a bump while on a stretch of swerving highway. Car nearly dove into the ditches, and it took a lot of effort to get the car back under control before it settled back down. $30 at a NAPA auto parts later for a new MOOG replacement and everything was good to go.
As others have mentioned, its pretty odd for a factory-stock 2WD front end to display death wobble, unless there's a component (or components) that are out of spec.
-- to this comment of yours:
Originally Posted by Supersporttramp
I was hoping to get some new rubber on my van before my trip but ran into some technical problems with the wheel/UCA geometry. I am hoping that may solve the problem in the future.
Is this wheel/UCA issue you mention something completely separate from this "death wobble" issue that you're experiencing? (I'm guessing the wheel/UCA issue is indeed a separate item, solely related to you wanting to put wider tires (and perhaps a new/different wheel offset) on the van, and running into issues where its going to crash the inside edges of tire/wheel into upper control arm.)
Might be worth investigating the integrity of all the ball joints/tie-rods/idler arms/pitman arms/steering box/etc with a respected suspension shop before going to a larger tire size. Larger rubber will usually only magnify problems that exist in a front end, not make them better.....