Sounds like a driveshaft center bearing issue to me. Especially if it is extended cab or 4-door and has a 2 piece shaft? I say that due to a u-joint issue gets worse with speed. Its not going to only do it till 25mph. The issue you are describing is usually caused from center bearing/isolator failure. Or improper driveshaft angle due to a suspension lift. If the latter is the case, the lift companies actually make brackets that mount between the frame and the driveshaft center coupling to bring the angle closer to its stock location....If is not lifted you may just have a worn out center assembly...
Last edited by 2000 Ram 2500; 02-23-2010 at 01:39 AM.