You know, it is funny but every Jeep I have ever lifted has had a shake between those speeds. Go slower, no problem. Go faster than 55 and it is fine as well. Typically it ends up being a TRE or loose trackbar being the culprit but I find it odd that it only seemd to happen in that speed range. Long story short: Check your front end components.
2003 Rubicon - 33x12.50x15 GY Duratracs, 15x10 MT Classic Locks, 2" BL, 2" BB, Warn Front Bumper, Garvin ATS Rear Bumper, Body Armor Extreme Rock Rails, Nth Degree Tummy Tucker, Skid Row Engine, LCA and Gas Tank Skid Plates, Teraflex Quick Disco's, Uniden Pro 520xl CB, 3' Firestik Antenna, BW Flat Fender Flares, Tom Woods DS, Currie Rear Adj Arms