94 suburban

have a 94 chev suburban runs exellent exept stearo was stollen during the summer so sat for a while whent to go and use it when going got up passed 35 to 40 mph its supposed to kick into the other gear but seems like its not not sure runs good never had any prob before seemslike when my stearo got jacked its been acting up any help i would be very gratfull trans dluid is full not burnt or nothing pretty sure and hoping its just something simple