What'I have a 2003 Silverado, 2500HD Duramax with Allison. I am getting a loud roaring noise and vibration at 45 -50 MPH then again at 65-70 MPH under load. Goes away when I release the accelerat

SOURCE: transfer case making roaring rattling sound

it's possible you might need to just change the trans case fluid.... it's a pretty simple task, i usually do all the small minor work first before i take it to a dealer to fix.... if they said you need a new trans case, but didn't tell you why, more than likely there might be something you could do yourself to make it better... those service men have been working around vehicles long enough, they should be able to tell you exactly what's wrong!
i would also do a tranny fluid change to... some of the noise could be coming from there too -- when you change either fluid, add a little lucas transmission fluid stabilizer, it helps alot with seals and makes the fluid stick to the gears better...

SOURCE: There is a loud noise at 45mph.

Find a quiet empty stretch of road. Go 50 mph, put the trans in NEUTRAL and shut the engine. If noise is still there, swing car left and right to detect noise differences. Could be hub or axle bearings. Not engine or trans if noise is still there. Hope this helps

SOURCE: 2004 Chevy, HD 2500, roaring in front end

need to check wheel bearings

SOURCE: My 2002 2500HD 4x4 front axle not engaging

Try jacking up one side of the front and putting it on a jackstand. Then turn key on(but dont start it) and put it in 4hi. Try to spin the tire in the air. The drive shaft should not turn nor the wheel. If the wheel does then maybe the acuator is not moving in the axle. It worked when you pulled it out but it might be hanging up on something in the axle.

SOURCE: I have a 1999 chevy silverado 1500 and I am

If the tires are round once checked on a balance machine to be true. Next check the driveshaft is a weight missing, dings or bent, u-joints loose?

Then check the rear ring