How can I offset and scale a differential voltage signal
I think a couple of voltage dividers will fit the bill perfectly.  Presuming a 'full-scale' input will produce +1.29 volts on one output and +3.80 volts on the other output (and vise versa for negative full scale), the difference between the two is 3.80 - 1.29 = 2.51 volts.  If you scale both output voltages down by a factor of 2.5 to get the differential voltage on the right scale, you get 0.516 and 1.52 volts, both of these within the 0 to 3.3 volt range.  So I would suggest simply building two divide-by-2.5 voltage dividers.  

Let's assume the output voltages are split into differential and common mode components.  In this case, the output voltages Vp and Vn would be Vp = Vc + Vd and Vn = Vc - Vd.  If you use decent voltage dividers, the output voltages to the ADC would be Vp,adc = (Vc + Vd) / 2.5 and Vn,adc = (Vc - Vd) / 2.5.  Then the converter takes the difference internally, Vadc = Vp,adc - Vn,adc = (Vc + Vd - Vc + Vd) / 2.5 = Vd / 1.25.  

Now, if the dividers are not exactly matched, then the result is a little different.  Call the divider gains K1 and K2.  The ADC output voltage will be Vadc = Vd (K1 + K2) + Vc (K1 - K2).  The differential gain is Adm = (K1 + K2) and the common mode to differential gain is Acm,dm = (K1 - K2).  In this case, the error caused by the common mode voltage will be proportional to K1 - K2.