Are you suggesting anything more complicated than below?

_.sortedLastIndex = function(arr, item) {
   var index = _.sortedIndex(arr, item);
   for (; arr[index] !== item; index++) {}
   return index;
};

If it work's for you all then that's fine (it doesn't return the expected result so would need tweaking).
I'm not sure how that implementation would do perf wise either but it's a start.

Usage example with expected results:

var a = [1, 2, 2, 2, 3];
_.sortedIndex(a, 2); // expected `1`
_.sortedLastIndex(a, 2) // expected `4`

My 2 cents in regards of @megawac code.

if item is not present in the array then it will go in infinite loop