6.2l Pulsing/vibration

HI,

I have a 90 6.2l 4X4 with only 33,000 miles. It starts/ runs excellent. When you get up to exactly 55 there is a slight pulsing vibration. It only apears from about 43-58. Its non existant at any other time. It dosent appear that there is a loss of power with the pulsing. I dont know if its an allison trans but it is a 3-speed. There also is no visible slop in the drive lines.

THANKS
-Anthony