I battled a similar problem for about 2-3 years, vibration only on acceleration. It first happened after I had a half shaft replaced. I took it back to get that replaced and it didn't help. Took it back to get the other one replaced and that didn't help. I finally ended up getting new OEM half shafts installed and my problem is gone!

1999 Contour SVT #1966/2760, Silver Frost/Midnight Blue Koni Kit, Mille Miglia MM-S rims, B&M shifter, Stazi mesh, SFCs, optimized Y-pipe & TB, SS break lines, K&N drop-in, Infinity Kappa speakers, MTX amp/sub