If the truck is one of the later models with the governor under the carb, 45 may be about all one can do without removing that.
It should be able to do 60 MPH, but it is at a fairly high rpm for the stock engine, close to 3500 RPM's, which isnt a good constant speed in most cases. The transfer cases also have penchant for developing super high temperatures that lead to seal and lube failure....the latter ruining the case. For any kind of distance driving, more than 30-40 miles of fairly flat ground at speed, 50 MPH has been shown to be a reasonable speed lmit to self impose on the system. More than that, or even trying to maintain that in steep terrain, leads to high heat and problems in the t-case.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!