Chock right-hand wheels, jack up left side of car (or vice versa, as you please), release handbrake, put little lever over to left and big lever in neutral. Then, as previously advised, arm yourself with pliers, WD40 and a bit of hope and get dirty.
Once you (hopefully) free it up, drop into difflock (and low ratio for that matter) from time to time to say "hello" or the box will go all sulky again.
If it won't free off, do a forum search and there's bound to be a useful set of instructions somewhere. Same tfr box as Defenders of the same vintage.
And find the rattle! Mine was crud in the handbrake drum.