Does anyone know if the 9.75 rear in a Lightning is physically the same as the 9-inch rear? Is it possible to swap them without major work? the 9-inch is plenty strong and has a lot more gear options available.

 

9" & 9.75" are two totally different units. Yes, 9" has many more available ratios & options for lockers, limited slip, etc.; however, 9" has no way to made ABS compatible.
If You installed a 9" in place of a 9.75", Your speedometer would not work. Signal is picked-up off of ABS tone ring (which is underneath the ring gear on 9.75" & 8.8".
I would advise against this swap.