It doesn't sound like the Uber benefit works with the Venture X cards. That said, Thrifty Traveler reports that Venture X cardholders get a free PRIOR membership (some sort of travel guide membership thingy.)

Weird that Uber isn't included with the Venture X, although I wouldn't be surprised if there's some sort of contractual reason with Amex that they can't do it (maybe the agreement with Amex covers travel cards, but not cash back cards?)