Thought you might be interested in a simple mod that we did on our 75 series TLC whilst travelling in Africa.
Our springs were sagging (and became a serious worry on the Kunene river road in Namibia). Met a guy with a 45 series who showed us this mod which he'd run for several years without hitch.
Simply cut approx 8 inch sections off the ends of an old set of leaf springs, and then insert the resultant wedges inbetween the springs, welding to the bottom spring only (thus allowing movement of the top spring over the bottom spring).
You can add as many as you need at the front and back of the spring.
We got a bemused welder in Windhoek to do ours for about £10. No-one believed it would work, but we gained about an inch and a half. We travelled another couple of thousand km on dirt tracks fully laden without any more sagging at all but with no loss of travel or shock absorption.
It's loads cheaper than fitting aftermarket springs, and the ride was still perfectly fine when unladen and back on tarmac.
The guy who told us about this mod claimed he'd never had to renew his springs after he did it. I guess introducing gaps between the leaves probably helps them slide over each other more easily which might explain the increased durability.