stuck in 4low!!

I just bought a 2007 z71 new body style 5 days ago and i put in it 4low and it got stuck and would not disengage and couldnt go faster than 15 mpg. My speedomonitor is also jumping around. I disconnected my battery and that didnt help. Ibrought it to the nearest dealership by my house. wondering if anybody has any ideas what could be wrong and how expensive it could be? i have the 100000 powertrain warranty but noy sure if it will cover it and i think i have a 30 day warranty from the dealership that i bought it from...ANY IDEAS what could be wrong would help!!! thanks

I assume you stopped and put in neutral to disengage, no?

yes, i tried in neutral, park and drive. tried everything. my speedomonitor is also jumping so thats what confusing me