Thanks for the reply.

A mechanic noted that the fan clutch bearing is going out. I tested it by removing the two belts on the water pump/fan pulley and that seemed to get rid of much of the vibration. (I think the current lack of a muffler is also adding a few extra rattles.) I had tried the same test before the turbo rebuild without getting rid of the vibration, so it looks like I may have had two (or more!) problems. I should get time to replace the fan clutch this week and will report back then.

I know I can see the timing belt by removing the front plastic cover. Is the balance shaft belt also visible or does it require further tear down?

Truck has no AC. I will go through the pulleys/tensioners.

I'm the second owner on the truck and I've now owned it for 10 of its 20 years. It has been a fun truck, though not without its problems.