Offroad-Express

Was thinking of changeing from 3.5 to 4.1 any one done that on a rangerover . I found the gap between second and third in hi range was a bit big and forth was very tall .

Yes, popular modification.
4.1 is aftermarket ratio and best option.
However in the old days we used to use Rover 90 diff heads in the series LR that were converted with Holden engines. The Rover 90 is only 3.9 ratio and can be difficult to find these days.
Even if you did find some, the 4.1 is the better ratio to use in Range Rover and Discovery if running 35's etc.

I did a bit of research on diff ratio's and strength... so I could sort the axles under my toy when I do the motor swop

Rover do a 4.1 standard diff... the catch was it was rated only marginally stronger than the 4.7 landy diffs, think its a very small pinion, I think there was a comparison by one guy "its the difference in shear strength between a block of butter at room temperature, and a block that has just come out of the fridge" lol

If you are going to a 4.1 ratio buy after market HD gears the Pom's were all using Ashcroft diff gears and axles, I'm considering Nissan Patrol axles or Custom toyota axles and hilux 4.1 the rover stuff is just too expensive to strengthen
If you go onto the Ashcroft site you'll find a strength chart they did testing all the standard axles, the interesting bit the Salsbry (sp?) axles did better on the rear but were weaker on the front think it was due to weak splines in the CV

KAM diffs offer good replacement ratios including 4.1. They were offering the reverse cut fronts long before Ashcrofts ever did.
http://www.kamdiffs.com/products-2/diff-ratio/

This is what was fitted to Larry LR and I have a set in stock if interested.