I had an issue last year with a similar problem. I changed my old springs and shocks for Terrafirma std height shocks and hd colour coded springs all round. It's only a 90 but I needed hd springs because of the heavy loads that I regularity carry and also for a future winch and bumper which will be fitted.
With the landy as std and only a roof rack fitted, the thing looked like it was pulling a wheeling with one side higher than the other - and not how you would expect, the pass side was higher than the drivers.(All the springs were on the correct corners).
Needless to say the springs were returned and changed, this time to some hd Flatdogs - std height, which have cured the issue and now its all level with or without a heavy load.