Chevrolet - Venture :: Front End Vibration Like Thudding Feel Of Out-of-balance Tire

I have a Venture with a steadily-worsening vibration problem which is always worse at the front passenger side. The vibration is not the growly rumble I remember from the bad bearings I replaced 3 years ago. It's more like the thudding feel of an out-of-balance tire, but the tires are relatively new and have been rotated and balanced since purchase and since I noticed the problem. The vibration shows mainly from 45-55 mph, but not if coasting. It's only when the engine is under load that I feel it. It occurs when the gear select is in D or 3. At any other speed it is not as bad, but if I get on the gas, the vibration becomes worse. Revving the engine in park or neutral reveals no abnormal vibration.

I jacked up the van and rotated the tires again and checked for any play in the bearings or loose mounting bolts - nothing obvious there. I did notice on the passenger side of the engine, around the inboard CV joint, a spray of thick axle/moly grease, like one would find inside a CV boot, as if the boot has a small hole in it, but the CV joint does not grind or make any typical noise on sharp turn.

My list of guesses include: motor mounts, tranny mounts, bad bearing/hub, bad half shaft/CV joint, tranny going bad, bad bearing in engine.

History/details: 2004 Venture LS long wheelbase. 160,000 miles. Goodyear Viva 2 tires. Front bearings have been replaced as a set once, driver side again since then, when the ABS sensor went bad. Sometimes when I put it into drive and turn sharply to the left, traction control turns on momentarily. The tranny has begun to slip and grab on a coast-to-stop followed by sudden acceleration.