01 3500 Tranny not Shifting
 

I have an 01 3500 it has been sitting for 16 months ,replaced batteries and it cranked rite up but the tranny wont shift up under normal driving if you floor it ,it shifts fine with no slipping when you slow down and it drops back to 1st it sticks till you floor the accelerator again ,when we parked it ,it was shifting fine.Any ideas????