How to change transmission fluid in z71 silverado

Have you checked the fluid level? Plus the color of the fluid. It could just need a transmission fluid & filter change.

Transfer Case
NVG 236 and 246 Auto-Trak II Fluid Gm P/N 12378508 NVG 261 and 263 Manual Transmission Fluid GM P/N 88861800

Sounds to me that you need your trans fluid changed and filter. When you do change it use a quart of Lucas transmission conditioner to it. Also check and see if you may have a bad univercel, It will also make it vibrate at faster speeds or your drive shaft may need to be rebalanced.

P0758 code is shift solenoid B electrical /2-3 shift solenoid circuit electrical, It better to inspect the wire going into the transmission they have a tendency to leak transmission fluid into the circuit make inspection for proper connection if you decide to replace the solenoid you still need to inspect the circuit for proper mating. If this solenoid fails and you wait to long to replace it you can loose the ECM due to the high power draw it takes from the ECM.

dextron 3 fill till runs out top plug

by removing pan and replace filter inside pan

change the fluid and filter if u havent already if that dont fix problem have trans. checked out u didnt say how many miles are on the vehicle?

my 99 z71 transmission lost drive and over drive. what could be the problem?

That was a good suggestion by Molson. Also, if you have been doing a lot of heavy hauling, have them check the strainer, which could be a problem as well.

Please check our motor oil. The symbol that looks like a transmission is actually suppose to be a little motor.