I definitely think the locked diff is a big factor there, but I have to ask a question. I'm not sure if I am reading it right. Did the car handle and turn into the corners very well and then slowly started to get worse, or did it start understeering suddenly after a part change (spring, shock, whatever)..

It's pretty difficult to know what to suggest without knowing a little bit more. Do you happen to know the front spring rate (being a mcpherson strut, the wheel rate and spring rate are close enough to be considered the same). Going too stiff will increase understeer but if it is too soft, going stiffer can help. I'd suggest that if the front spring rate was under 150lb/in then you could safely run stiffer.

It might also be that the shocks (original fronts with new oil) have started to get 'sticky' (I wish I could think of a better word) - and they aren't allowing enough movement, and it's making the front easy to slip on initial turn in. Maybe it has slightly blocked/restricted some of the valving on the shocker. Maybe try emptying them and replacing with thinner oil as an experiment (it might end up being too soft then, but it's worth a try).

Very generally you can do a couple of things to help intial turn in understeering. Of course the 'good old' option is to run more negative camber on the front wheels. I think it would be quite hard to get too much front neg camber - there just isn't enough room to move the strut top inwards (unless you run a longer control arm).

The other thing that works, but I have to warn you, it will wear out the front inside edge of the tyres if you run it on the street or very long distances. But the trick is to adjust the front toe-in - you adjust it to provide anywhere from 1-3mm of toe OUT. This typically helps initial turn in but it isn't as good for long distances, and can be a little less stable/smooth at high speeds (compared to 1-2mm of toe-in

About the only other thing I can think of (for now) that might be a factor - is there any chance the front tyres have 'gone off' - this can happen if they are massively overheated during races - even if there is a lot of tread left. Another possible cause would be if they got a lot of oil contamination or something.