I would agree, that sounds more like a potential wheel balance issue than transmission or anything that would need a computerized scan... depending on the type of vibration. Unbalanced wheels will start to become evidence at higher speeds. Can you tell if there any "wobble" to the vibration, or is it more like a loud "buzzing" sensation? Those are the best words I can think of. Feel free to describe it otherwise if you can!
If you had the tires/rims mounted & balanced somewhere recently, drive back in and ask them to re-balance the wheels. If your wheel weights (small curved metal pieces, hooked into small brackets on the edge of the rim) are on the outside as most are, also check and make sure none of them flew off.
Current GMs: '95 Chevy Suburban C2500, '69 Pontiac Executive
Former GMs: '83 Buick Park Ave, '73 Buick Electra, '92 Chevy Caprice 9C1, '98 Pontiac Bonneville SLE, '99 Olds Aurora, '00 Buick LeSabre