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I use Aldan coilovers on my Jag rear in an Everett Morrison. After 42K miles I'm on my third set (which is actually the second rebuild of the first set.)
That said they're not really the first set - those were too short and were replaced when nearly new. The too-short shocks made driving on a track interesting - in a high speed turn the inside wheel would pick up off the ground and cause the rear of the car to skip sideways. An inch longer made them just right.
Here's another point: it will depend on the wishbone length as to what's the correct shock length. Stock Jag wishbones have three lengths: XKE is the shortest, 3.4S/3.8S (and maybe 420G?) are the medium length and XJ6/XJS are the longest.
As the wishbone length varies, so might the location of the lower shock mount. If that dimension changes, the correct shock length may vary too. Changing the location of the upper mounts can also change shock length.
So take into account those factors for your car and determine the correct shock length by moving the wishbone through its normal travel and you'll get it right. There are many instructional links on the internet such as this one:
https://www.qa1.net/tech-center/tech...asuring-shocks
Once you have the length and stroke, you will be able to order the correct shocks from nearly any vendor.
Spring stiffness can be measured. You can decide at that point whether to go up 10%, 20% or whatever to try to get your desired rate.
Hope this makes some sense!
Tom