Should a toe-brachial index be obtained in lieu of resting ABI as an initial screen for PAD in high-risk patients such as those with longstanding diabetes or advanced age with stiffened vessels?  

Yes, a TBI should be used instead of an ABI in patients with diabetes and chronic kidney disease as the ABI is likely to be inaccurate due to non-compressible vessels. An arterial duplex and TBI should be the test of choice in this patient population.