Hi Rae,
one thing that I noted is with respect to your 2k RBIAS. When using 250uA of excitation current your reference voltage for the ADC is 'only' 0.5V. That is the minimum the ADS1248 allows. I did some rough calculations (using attached calculator) and it seems to me you would get slightly better resolution when increasing the reference voltage (either by increasing the current to 500uA or by using a larger RBIAS together with higher gain - or all of them). The accuracy of your system will not improve, just the resolution and that you do not operate the ADC at its limits (VREF=0.5V).
Do you only need to measure temperatures up to 60°C?
Regards,