max speed in 4wd

Check your owner's manual, I have been well over 120kph in 4hi in my Power ram to no ill effect, depending on the 4wd system you have in your Jeep you can use 4hi on pavement such as the high way, witch means high way speeds are not unreasonable. However most 4wd's are geared diffirent front to rear i.e. 4.55.1 rear to 4.56.1 front, thus causing "bind" in the drive line.

In answer to your question, some manufacturers (ford, gm, dodge) only recommend speeds up to 80km/h (close to 50mph) in 4hi. sure, you can drive faster, but this does cause premature wear on driveline components. I know for a fact that GM does not recommend driving over 80km/h - besides - if you need to put it in 4wd on the highway, you shouldn't be driving that fast in the first place! Not sure where Charlie got his info on the different gearing front to rear, but no 4WD system has different gear ratios front to rear, if that happened, the entire system would always bind and eventually break or damage components (like stretch the t-case chain)

I assume that your jeep is all time 4wd. In my 2000 Jeep Grand Cherokee Limited V8 which is all time 4wd the governer will kick in at about 115 mph.