Response 1) If this is an automatic transmission, it could be as simple as a VSS (vehicle speed sensor) or a failed shift solenoid. Neither are too expensive part wise so I wouldn't think the dealer would give you much guff over it. Besides, a deal is a deal and they said thy would fix it....... no worries.
Response 2) 1. Why can't you check the trans fluid?
2. Perhaps your Prndl is off
I drive an old Dodge Dakota and the prndl is off by one step. Drive is in neutral, 2 is drive etc. The little floss sized string has stretched over the years.