on my 01 ctd i get this feeling of slop in the drivetrain but the odd thing is i can start a slight roll all is fine, and by this point you would think all the play would be out and when i get on the gas it gets this jump feeling like i have massive play in the drivetrain. ive had on a rack and all the play is within manufacture speck. this is driving me nuts since i cant figure it out.

 

Check the engine mounts.