Well even with a perfectly working system XJ braking leaves much to be desired. My finance has a 2010 Ford and I do the same thing with the pedal, whenever I'm driving and that first stop comes everyone gets whiplash
But you're sure everything is in working order? Checked master cylinder, lines, calipers, rotors, pads, fluid, etc?
I just replaced the pads on my 96' and found that there was a small leak from the piston on one caliper. Replaced both, bleed it well and went on my way. My brakes feel a million times better now. I didn't even realize how bad they were before.
So first I'd check that there are no leaks or other problems and then if it still seems bad maybe upgrade the rear drums to discs?