Spongy would be a "soft" (or softer than normal) pedal. As far as vibration, something sounds out of balance. From your description it's only under load & speed which makes me think it's a balance issue. Maybe you picked up something or threw a weight off driveshaft. I wouldn't get too stressed unless it starts getting progressively worse. It could be something as simple as a transmission mount. 2 things to look for: A) Trans mount worn or showing signs of wear (cracks,etc..) B) a stone, rock or something wedged under mount. I had a stone lodged under trans mount and could feel all kinds of things in pedal.
Could also be a support bearing issue (if your ride has one - I'm not in front of one to know) Keep me posted. I'll check some of my factory manuals when I get home. FINALLY (Haven't been home for 7 months)