I'd say take a look at u-joints. If this is the same type of noise like I am getting from underneath, then it's not tranny. It comes on suddenly while coasting, and may go away if you just press the gas a bit more? Then it will kick in again when letting off the gas, and go away when you reach below certain speed?
I just had a conversation with mechanic who specializes in driveshafts. He is going to disconnect my front driveshaft to see if that's the culprit. We were thinking about u-joints because of my lift. If not, I'll keep looking.