You don't need to install the SYE first. You can measure the Jeep the way things are now and order the SYE and drive shaft all at once. See the photo below. We also have this photo amongst the product photos on the TJ double cardan rear drive shaft page on our website.
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Build time will typically be one day on a drive shaft, you order one day, we ship the next. Exceptions to this might be Mondays because we get a ton of overflow orders from the weekend. Or if you place the order after 5:00 pm mountain time, in which case we don't really get the order into our system until the following business day. If we end up with a full list on Monday afternoon which is pretty typical or if you order after 5:00 pm any other day, the order will roll over into a 2 business day build time. Once we build and ship it you can find transit times here
https://4xshaft.com/policies/shipping-policy.
But again, you can order the drive shaft before installing the SYE so there's no reason you need to have any extra down time.