Tires. I did tires. First I pulled the Kelly 255/85-16s (33") off the '95, then pulled the smooth riding Michelin 205/80-16s (29") off the '88 and installed them on the '95 to test if the vibrations I was feeling were from the tires or something else. Answer: both. The tires definitely helped up to 80 mph but there are still some vibrations in the driveline when 'floating'. Then I put my 225/75-16 (29") BFG MTs on the '88 and gave it a test run which was louder and harsher but OK. Now to pull the Michelins off the '95 and put the Kellys back on. Or maybe I'll leave them this way and take the Kellys to a tire shop to have them checked out.