Correction needed Brock, your a woman or lady and not a girl respectifully speaking.
I'm surprised there was a spring conversion replacement available vs dealing with airbags unless airbags were an option and coil springs as standard equipment?
Considering diesel engines weigh more than gas engines the diesel versions having stiffer springs compensating for the added weight. In your case I bet your diesel has stiffer springs?
Have you test driven another V6 turbo diesel to compare yours against other diesels your harsh ride?
I'd be looking into aftermarket lighter or softer springs once you know the specs of your harsh riding springs able to select softer replacement springs if they are available?
Also look into replacement shocks as they may be your harsh riding issue vs the springs?
More tech and features the more to go wrong or fail considering Land Rovers started out as simple yet solid vehicles that were trouble free. Underpowered especially the diesel I recall topped out around 53 mph best.
Two family friends had LR's dad and I gave them annual tune ups, a 54 and 57 Rovers they owned for decades.
One "watch this" adventure the 54 rolled ending on it's lid. Block a tackle back on it wheels again. Added oil replacing oil drained out while upside down along with battery the acid drained out, guess what it smoked, cleared up and back running normal again. Body dents plus broken windshield running another four decades at the ranch.
Basic simple not a full on electronic nightmare requiring dealerships to keep running everytime the LR decides to fart out of tune.
If I were way out in the bush with zero outside support i'd take a 50's era LR be it diesel or gasoline powered with a carburetor over anything controlled by electronics and computers.
Plastic id radiator burp plug know to become brittle and break stranded, happene me with the 95 D1. Threaded in a green stick of oak a plug and out from 23 miles deep in dirt years ago. Plastic LR cutting corners, a early Range Rover with a bronze burp plug a nice upgrade along with a spare O-ring just in case along with a spare distributor, alternator, belt and plugs. Tyre plug kit along with 12 volt compressor.
You mentioned coil springs vs airbags, i'd check into the wire diameter off a coil along with spring outside diameter, length relaxed not compressed plus the number of coil turns.
Enter these numbers into a good spring site you will get the lbs./inch or if into metric Kg./ mm numbers for the spring rates able to compare.
It would be a challenge to compare your coil springs vs airbag "firmness or stiffness" vs different coil springs easy to compare your spring measurements able to know if your springs are way stiffer vs normal springs what is available for your LR?
Sounds like the PO replaced airbags with springs that's causing your bone jarring ride.
Any way of contacting the PO and finding out your spring manufacture along with part numbers, find the spring rates as a starting point when replacing with lighter springs?
Contractor friend has a 2005 "Super Duty" 4 x 4 F250 truck, a city street mile ride is enough for me a bone jarring uncomfortable ride. Stiffer upgraded springs on the 95 D1 still has a comfortable ride allowing 500 miles a day over several days on end not having issues forcing yourself into the Disco for another 200 miles driving.
Unless your going to catch air and beat the snot out of your LR i'd install weaker springs if it's that bad and it's a "keeper" vehicle.
Did the PO built it up for hard off road duty preventing bottoming out the suspension? Been abused comes to mind where serous off road is more important than comfort.
I have "Heavy Duty" stock heigth springs and shocks all around on the 95 D1, firm but comfortable being street and highway only that increased cornering abilities by a large amount along with 190% front & 230% rear stiffer anti-roll bars. If left factory original this D1 would of been on it's roof a month after new ownership back in 1/2000. A ugly surprise happened once holding a line at app 48 mph going into a turn that tightened up, had five to six seconds up on two wheels. Right then and there stiffer springs and anti-roll bars required plus removed all the rubber suspension pieces, installed all Poly bushings everywhere possible.
Steering sharpened up, responds with small steering corrections, no wondering along with correcting steering working the steering wheel constantly instead just held in a fixed position.
Loose steering box, tie rod ends and ball joints, don't forget loose wheel bearings, shot rubber leading and trailing arm (radius arm) bushings resulting in wondering steering turds to drive Land Rovers. Playing submarine rused and worn out suspension ball joints and A arms of newer LR's. Solid axles at both ends no worries properly maintained swivel bearing preload on the older Disco One. alignments set and good for decades.
This is 100% out of my intrest in LR Discoveries, i'm geared up with specs concerning 94 and 95 US imported Discos with the only addition and details of the 4.6 litre engine along with aftermarket upgrades along with a chip to wake up those engines yet able to pass Kalifornia smog checks legally. Under two minutes remove cold ram air ducting, rotate one sensor to factory spec for the visual and tailpipe sniff inspection. Readjust, reinstall cold ram air ducting back to good running condition again vs other LRowners that have to replace their performance engine with a factory engine every two years to pass those nasty Kalifornia smog checks.
Sorry i'm out of ideas, all the best in solving your harsh riding issues Brock. .....~~=o&o>.......