vibration

[ab318i]

Have a 1998 318i 5 speed with a vibration at highway speeds over 40 mph. I have replaced the flex disc, center bearing assembly and harmonic balancer that applies only to 4 cyl. eng. on the drive shaft. I still have vibration. But when I take my foot off the gas the vibration goes away immediately. The vibration is in the seat of my pants not the steering wheel. Any ideas. I could really use some help here. Thanks, Allen