Need a SYE?

Hi, 91 XJ with 8" rock ready lift, have a quick question. I never get any vibration in my driveshaft or anywhere, and I have no SYE kit on my T-Case. it's a selec-trac. however, when i slap that bad boy into 4wd full time, i get massive vibration if i go over 10 mph. does this just mean i need a sye kit or is it something else?

I didnt even read past "8 inch rock ready lift"....

Regardless of what the next sentence or two says, YES you should purchase a CV driveshaft.

first I am assume you have an open front diff with the disco front axle and if this is correct then check your front driveshaft for wear or binding this setup will allow your front driveshaft to remain stationary in 2wd and the moment it has to turn you are feeling the vibes?

Definately sounds like a front driveline problem if everything is fine in 2wd.