Official top speed is something like 98MPH I believe. It would be scary to drive a lifted Jeep that fast though.
The problem you have with top speed is that it changes with gearing and tire combination. Not to mention aerodynamics. Still your jeep should be able to do 75mph fairly easily on a flat road.
55mph at 4000Rpm's is a lot in 4th. I'm really wondering about the accuracy of the speedo and/or the gearing of that Jeep. Can you crawl under it and look at the differential cover and read the ID tag on there. It should be a metal tag at the 100'clock position. There should be some three groups of letters and numbers. What I'm looking for is teh middle set. It should read something like 307, 354, 373, 410 or 411.