With the 4" lift you have ventured into 'more things to change' mode on your YJ. I put a 3" lift under a 94 YJ and with the 4 cyl. there wasn't enough weight to 'compress' the springs so actual lift was over 4"! But we had anticipated that and put in an SYE (slip yoke eliminator) for under $300, and had a spare driveshaft cut and sized to fit for another $100 I think. Just sold it 2 months ago and had only used it a half dozen times but it kept up with my Rubicon on every local trail without lockers! And no vibration or problems with ujoints.