OK, so I changed all the gear solenoids for new ones. The 2-3 shift is very slightly unsmooth. However, I now sometimes go into limp home (F4) mode when the gearbox is cold. I have had the codes read and it show that ratio 2 and ratio 3 are out of range. The mechanic who read the codes said that he also believes the viscous coupler has locked up. First of all, is it possible that the viscous coupler failure could contribute to either the slightly unsmooth gearchange and/or the 'out of range' error? Is it new gearbox time?