That sounds more like the hubs than a driveline or transmission problem. If it were transmission it wouldn't go away by taking your foot off the gas. I had my trans rebuilt a few years ago but it never made a hum or vibration of any kind, it would slam into gears pretty hard tho.
I've replaced the hubs on the front at least 3 times in the 180k I've got on the Sploder now. All 3 times it sounded exactly like what you've described here. Once, the problem was compounded by a blown belt in one of the tires but after the tires were all changed (bought a whole set at the same time including spare) it sounded for months like I was running on knobby tires. Turned out to be the hubs. That time I had them replaced at the stealership which ran $1200 for both sides.
The last time I had the hubs replaced I had a shadetree mechanic do them for me the parts were around $160 ea side (with ABS) and I paid $150 labor. Total cost with a few side jobs thrown in didn't cost $500 total. Much better than the dealership.
I'm sure someone on here can tell you more about how to go about checking, testing etc., the front hubs, I can only tell you what it sounds/feels like when they go bad. I experienced the same hum (deafening at some speeds) and vibration (constant like a massage chair) as they went bad. Luckilly I never blew them out completely and did more damage.
Just had a problem with an 87 Mazda 626 where the rear bearing completely dissintegrated and left the bearing race welded onto the spindle. Now instead of $25 bux in bearings, seals and grease, I need a new spindle $120 and brake drum $60 and just a little more labor (my own) to finish it up. Catching bearings early is DEFINITELY worth the savings later.
Do a search or solicit some replies on what and how to check the front hubs more definitively (I'm not a mechanic) before running to the parts store on my word.
However, I hope it helps a little.
Matt
