Have a look under the hood and under the rest of an XJ, once you look close, "frame" won't be a word you use.
There really is not one to be found.

Semi float: the weight of the rig rests on the axle shafts.

Full float: the weight rests on a wheel hub/spindle and the axle shaft bolts in independently.

Full float D60s don't EVER come stock with 35 spline shafts.
Rather, they come with, weak, 30 spline shafts, not worth running.

Both can be built to be bomber strong, semi float less expensive to do. The advantage of full float is if you do break a shaft, the shaft can be removed without even taking the tire off.

SD