rattling and vibration

[crash_41382]

A few weeks ago, my 2000 ranger started vibrating slightly, we thought it was the tires and figured i would get em balanced when i got the time, then a couple days ago i noticed a rattling noise coming form underneath. i went on and had the tires balanced and rotated, nothing changed. tonight i heard a knocking noise coming from underneath, sounds liked a cracked flywheel but not squealing and it doesn't do it all the time, the rattling or the knocking sound. i got home and tryed power-braking it enough to put a little stress on the drive line to try and figure where the noise was coming from, sounds like a flywheel or tranny problem. also the vibration is worse around 60-65, and real bad when decelerating. its kinda like a vibration with a rumble, i mean it wont shake you to death but you can definitly tell its there. i put new u-joints in about 4 months ago when i changed out the rearend.

any ideas on this, its got me stumped.