i have a 90xj just bought a 4" lift for it and a hack n' tap sye and driveshaft kit. im just wondering if i am actually going to need the sye right away or if i can drive it for a while then put the sye on it later?

 

no u dont need it right now i would install a t case drop much easier

 

will there be any vibrations? and will it damage it when it flexes?

 

Test it before the SYE is installed. Should no damage anything. Just make sure that the yoke goes in far enough.
I have just over 3" in the back with stock d-shaft and no vibes. 89xj aw4/np231

 

Hey guys I have a 4.5 rough country lift on my 1990 wit 33" 12.50 all terrains and i was wondering if I should get a slip yoke. I bought my jeep lifted so idk if have a t case drop or what. And what is a hackk and tap ?

 

hack and tap is modifying the existing transfer case rear output shaft in order to use a fixed yoke.
http://www.angelfire.com/nc3/84jeepxj/sye.html
check your transfer case cross-member. if you have a t-case drop, it's gonna be a 1" block between the frame and the cross-member.

 

I have a 92 with the aw4 and np231. I have 6.5" of lift with a tcase drop, and 4 degree shims on my axles, i have no sye, and a stock driveshaft. I have no vibrations, and no binding, even at full flex or full droop.