My 2004 Envoy (89,000 miles)recenty started making a sound that appears to be Auto Transmission related. it shifts in the lower gears fine but when third gear shift there is a slight vibration in the steering wheel and a gear type humming sound. As I slow the vehical doen the vigration and humming like sound decreases. Fluid level is ok and normal color... Any Ideas ??

Yes, most likely. The linkage needs inspection from the shift lever to the transmission. You may have to check under the vehicle for disconnected linkage AND under the cover of the shift lever inside the truck.

Sounds like your blew your tranny.

The valve body has failed inside the transmission. . It's possible the shift solenoids or the transmission control module are faulty as well.

Those would be some of my first guesses as well. You could begin to eliminate some of the culprits by flushing your trans. Take it to a quick lube or somewhere similar and get it flushed. Understand though, that if it's a fatal issue, and it's hanging by a thread, flushing could very well send it over the edge. I would highly recommend a transmission rebuild, as after 160k+ miles on a vehicle will put any transmission through it's paces. But flush first, and see what that tells you.

its most def a purely transmission issue i believe its the solenoids. have it taken to a reputable trans shop near and have them diagnose it with there speciall code reader.

Sorry to say but your mechanic is probally right because 204000 miles is alot of miles for a transmission and when the clutches start to slip thats what it sounds like.

May be the alternater tring to recharge a weak battery, has it been starting ok? Have the battery load tested.

The brake rlease switch for the gearshift might need adjusted-replaced or the shifter itself might need adjusted

My GMC 2004 Envoy when in drive will not shift out of 1st gear. When driving the Transmission doesn't sound like it's slipping. Does anyone know what the problem might be?

Do you have a CVT? (Continuosly Variable Transmission) I doubt that you do so if you dont, nissans are known to have hard shift into second gear you mean.(Once the car moves, its already in first gear) so if it shifts fine into other gears, I wouldnt worry too much unless it gets worse.