not sure if there is any MPC5748G example using legacy descriptors.
You can refer to AN4577 (https://www.nxp.com/docs/en/application-note/AN4577.pdf ) and its SW package. It is for MPC564xC but the module is very similar.
Also you can look at below thread, where you can find some useful information about ENET configuration. There are some examples too, but enhanced descriptors are used, I think.
Finally, check if RX/TX descriptors and buffers are properly aligned (128-bit aligned, i.e. evenly divisible by 16) and whole placed in non-cacheable memory region.