Yes I did it to a Pajero when towing boat and now to Landcruiser Ute towing van.

Petty simple go to spring works is usually the cheapest,,, not retailers.

Having owned a Territory Ghia/Titanium (and my two neighbours work at two different Ford Dealerships) I was surprised at just how much the terry sagged with very little weight in the tow bar.

The suspension is not tuned/designed for towing caravans, nor are they happy with having 150kg+ on the towball.

Mine sagged over 2 inches in the rear just when hooking up. I used HR level ride bars yet over 15,000km that sag became permanent.

I quick visit to Pedders and $900 later had terry fitted with heavy duty springs and new struts.

The difference was amazing. No more tail wagging, and braking was much nicer without the front ABS/ESC/traction control thingies kicking in all the time.

Unfortunately 6 months later terry was sandwiched between a Nissan Murano and an overloaded tradie in a Holden ute towing a tandem full of sand who couldn't stop.
$80K of vehicle and because of their horrible resale values the RACV payout was a measly $16K. 

PS
Whilst the diesel versions have a slightly higher tow rating over the petrol variants this is primarily to do with the diesel engine having less power (kw) and therefore applying less stress to the transmission.

Spring packs normally go up in 300\400 kg steps.
and Supercheap, etc. will sell models to suit most veh's.
Just remember to ADD bush set Pricing to cost.
It can nearly double it. Depending on the bushes.

AGAIN.. Alternative.

IF you happy with the veh's suspension without the van on rear.

Little things called "Helper Springs" do a good job,
as long as you not needing to hold up the back of a truck.

My D-Max rears are leaf. sag out. Bang a bit with the Tandem van on back (With HR)

I have a set of "Ironman, Half leaf helpers" on rear of my second ute
They ARE good And a lot better than those Airbags that DO bend a lot of chassis..
I've seen them as well as read about it.

Set the Helpers to float IE. NO load. when solo. But van weight lowers rear enuff
that the helpers come into play. Then back to normal again when van not there.
You don't have to take them opff
And around (were) under $200 a pr. Oh. I find the HALF leaves better than the FULL leaf versions.


ALSO. Again.
Older veh. Done a few miles. OE Shockers are NOT the best things in creation.
They've probably crapped too and need changing.
I change mine when veh new and straight home from dealers.

Spend for a set. x4.. of Next step up. with 1 or 2 in lift on them.

Several reasons.

The one step up. Are stronger. Hence, MORE resistance to piston movement.
Both slowing down the bounce, and help to hold up a little better.

The 1 or 2 in extra travel gives your suspension a little more freedom,
in overall travel. and can (does) help a little in their travel.
On Aust roads and tracks.

Just buying leaves will probably still have you forking out for shockers as well. Sorry.

They actually dead easy to do yourself with trolley jack, coupla axle stands.
some grease and a (Probably) bench vice for pressing bushes.
Some do. Some don't.

YOU TUBE tells you ALL.
Just have a look.

Ford terry uses coils all round so half springs seem pretty useless.

Using a qualified mechanic in doing the work is far better as he is trained in all aspects of mechanical then sales person at ie Pedders or similar store that sells there own products.

Myself I would look at air bags inside your coils. Your car with 120000ks is probably looking for a major make over on your suspension using better quality products equal to what you are doing will give you more satisfaction then just replacing the rear springs.

Check and get quotes from several suspension houses or workshops.

We uses a friends son who owns and runs a workshop using the lastest gear, services our cars when I say to him how about we use this or that his said that may of worked on your old Holden FJ but want work on your modern Nissan. Less is best. He is young has great practical workable ideas and knows where to buy with quality, not like us that are getting on a bit.

Hope this helps.

Used king springs and new shockies as well as air bags in my RW territory, made a big difference. Recommend you take it to a specalist in wheel alighning after to get it done properly.

My van is only 1350 atm but the terri carried max weight great improved. I also put my tyre pressure up to 40 psi when towing on the black strip on recommendation from my tyre bloke. Have a 2012 Titanium with 157000 on the clock and very happy with her. Cheers