I completed the repair and can report on the results now. I ordered a new tensioner spring, shock absorber and idler pulley just to have everything I would (most) likely need on-hand.
Starting the job, I intended to replace the shock absorber, the spring and the belt. Really the shock was not in bad shape, as I found the new part had the same characteristics as the one on the car. Hard to compress but easier to release. Looking at the shock on the car, hmmmm, that takes some work to replace - I'll just do the belt and the spring.
So, after the new belt and spring were installed its all smooth again. Although two components were replaced, I'm pretty sure it was the belt. On inspection, not only were all the interior ribs all cracked up, there was a small notch missing out of one side. I suspect this was contributing to the vibration as it passed some pulley.
The manual says to push up a loop of the belt between the crank pully and the water pump / fan pully to hook around the idler pully. (They tell you to turn the fan pulley anti-clockwise to assist.) No way are you going to push a loop up between these two pulleys - not enough clearance. You have to make the loop, push it up well past the gap between the crank and fan pulleys - then push the belt into place on these pulleys. That is, you push the loop through while in FRONT of the pulleys, then slide it into place. At that point you can immediately complete the loop over the idler pulley and continue routing the belt elsewhere.
Overall, not as hard as it looks. I did have to refer back to the manual to get the belt routing correct. If you don't have a manual showing this - please draw all the pulleys and the belt routing first before taking off the belt.
Ken300D