When driving my 2000 Olds Bravada, it has a loud roar noise that sounds like when a semi truck is taking off before the gears are shifted. Transmission shifts easily into gears, just this loud roar noise all the time.

have the OBD read for any fault codes

Bad front wheel bearings will make a loud roaring sound, and get louder as time goes by, check this out! God-Bless!

universal joint

Do you have reverse.?

A loud "roaring" noise as you describe it could very likely be a bearing going out.

Have a shop look at it. It may just be a failed wheel bearing. The bearing will leave you stranded if it gets bad enough and cause more damage.

Try making tight turns to the right and then to the left. see if the grinding noise comes and goes. It could be your C/V joint and they need to be replaced. The noise will only come when your making the tight turns, you don't have to go fast. Walking speed will do and you can hear it with the window open or someone walking beside the Bravada. Good luck and keep me posted. Be glad to help.

you may be low on gear oil...have someone check it for you before the rear end locks up

AWD should not be effected, but sounds like you do have problems.

What do you mean by loud... Loud where??? If noise is from engine, make sure that the mechanical fan clutch is not "locked" (it's supposed to freewheel when cold... if it does not, it will "roar" when driving.