I wonder, could it be the idler pulley?
Try putting some extra pressure on the pulley with a pry bar when the engine is running.
Had a friend who had a 91 Lincoln that was his problem.
Also pull the pulley back and if it the bearings are gone you will feel it when you spin the wheel.
That was the problem with the Lincoln. We replaced it and all was fine.
My 06 Colly has a creaking noise like a worn out hotel bed when you first put it in gear and move at slow speed over bumps or turn corners. It too is only adible up to about 25mph. I read on the colorado fans web site that some others there had had the same noises & that it was determined to be the suspension bushings in some cases & the cab/box mounts in others. Haven't had time to get mine to the dealers yet, but it's definately going in. Hope this helps.