When balancing, did the garage put the buckled wheels on the back or were they there anyway?
Are the vibrations at specific speeds, does it do it whilst accelerating or just cruising, is it only in certain gears, speed related or rpm related?
Trying going around 1500rpm in top gear, floor it if it shakes around like a very shakey thing, it's likely to get the other driveshaft. If you jack up the car and take a look, you should be able to feel play in the shaft or if it's a split in your rubber boot, black grease will be everywhere.
After I'd done my driveshafts, the vibration was being caused by the balljoints in the steering rack wearing due to water getting in and washing out the grease. I'm in the process of sorting this out now.