since when do you need to install adjustable uppers to put in a SYE? first, you never want the pinion pointed directly at the t-case output, it will not facilitate the u-joints needle bearings to move and lubricate the trunion, you want a couple degrees of deflection, plus as mentioned, you will starve the oil slinger in teh diff of oil and cause a failure. Also, you can install a SYE with out adjustable uppers and add the uppers down the road. furthermore when you install a SYE, you are increasing the length of the Drive shaft, which in turn changes the angle of the shaft at the pinion u-joint which will provide a more ideal angle than what currently exhists, so if you've been driving around with a slip yoke and a XX lift, adding a SYE and longer driveshaft alone would start to remedy the issues of vibrations. I dunno, this is the first time I have heard that one needs to be done because of the other, but I also have not heard a lot of things.
Jesse, I personally have not ordered from them, but I know people who have and have not had any huge issue.
