Just bought a 2001 350 Crew Cab Dually with 140K miles...I noticed this same grinding noise in 1,2 gear while coasting thru a parking lot....drove it a good bit more and listened at different speeds...only happens with little to no load...Gear lash is correct. I took it to a mechanic before purchase and he confirmed that these things just make noise and theres nothing you can do to fix it.
It is annoying but i just try to keep the rpms up so it doesnt sound like a beat up old truck....
