Touchy xj

I just spent 9 hours trying to correct driveline vibes....to start its a 99 I also have an 89 and 92 both lifted and were pretty simple to lift and aleviate vibes. But this 99 is so touchy...I'm lifted 4 inches so vibes were outrageous, installed a SYE and they got worse!!! Used an angle finder and to the best of my calculations I was about 4 degrees below 0 so to close the gap I decided to put in a 1 TC drop (really didn't want to but given the sitaution) Tried again and it vibrated like an earthquake. After scratching my head and screamin' I just grabbed at something and decided to use 1/2 inch spacers instead of 1 inch and it drives like a dream..........so finally my question, why is this one so touchy? Am I missing something obvious?