Some questions about my new lift

I have a 1996 XJ 4.0 AW4 231 that I just got back from the lift shop yesterday afternoon.

I went with the RE6300 5.5" long arm kit, 33"x12.5" Truxs MTs, AR767s, slip yoke eliminator, Trac-Lok rear, and an ARB air locker for the front diff.

What tooth speedo gear will I need?

Also I can hear a loud humming noise when decelling between 45 and 30mph. If I slip the trans in neutral, the noise still continues. It will only happen around those speeds. The noise similar to the vibes I got when I installed a 2" budget boost. Does this sound like driveline vibes or tire noise?

Thanks for the help

here is a link to gojeep's website for calculating which speedo gear you'll. there is a lot of useful info there, so read up.

http://go.jeep-xj.info/HowtoSpeedoGears.htm

My guesse would be the tires making the noise but it should hum all the time not just at those speeds so Im not quite sure, It could be the driveline like you said tho.