here's some ideas
Well, if you want, you can fix the one you have if it isn't too bad. Once you have it done, maintain that t-gate inside and out- religiously! Wash your truck alot too, and spray out all the salt and dirt and junk with a hose, under the wheelwells, and the back of the lip on the bottom of the tailgate that rots and stuff.
Drill some drain holes in the bottom of the tailgate on the inside as far to the bottom as possible to keep the water from collecting. It may be hard to make the holes, but nothing is impossible, I think you have to open the tailgate and do it where the hinges are or something. You'll figure it out!
That is the reason these things rot, the weatherbelt in the back window gets deteriorated, and the water leaks in to the bottom of the tailgate and pools up, and rusts a way for it to get out! There are no factory drain holes in there (Damned Lee Iacocca and his theory of planned obsolescence!!)
Once you get it fixed and the holes drilled, spray the whole inside of the shell with undercoating or POR-15 paint, a spray-on bed liner or something like that.
Make sure there is no rust under the paint though, it will just rust again. You should put naval jelly on all the rust or sandblast the inside and prime it first, then spray the thick stuff on. You only need to coat the bottom, but if you want you can do the rest, just dont get it on the mechanical "organs" or the glass! Afterwards, put rubber grommets in the drain holes to keep the edges of the from rusting. Every so often, spray lubricant into these holes too, WD-40 is good.
If you should decide to get a whole new tailgate, get ready for a pain in the a$$! It costs a lot of money for the new shell, and you have to use all the old worn out mechanicals from your old one. JC whitney might have a cheaper one, but it may be a mickey-mouse poor quality one. It all depends on your vehicle year, the condition or your current t-gate and inner parts, and the probability you'll just smash it while off-roading, or how cheap you are, lol.
If you have a newer Bronco, get a new tailgate. If you have an older one, like your 83, get either a cheap new one, get one from the junkyard, or fix the one you have!
I'm surprised that yours has JUST BEGUN to rot, it being an 83 and all. My dads 85 was completely rotted through on the bottom by 1989! I've seen 94's with rot on them too. We've all been there!
IMHO, If it's not too bad on yours, I'd just repair it, you can always get another one if all else fails. Hope this info helps you out.