There is no limit to the speed you can go in 4wd. If the "hubs" you are talking about are the lockout hubs they are not your problem, their only function is to connect & disconnect the front drive axle to the front wheel. There is nothing within the hub that can cause the problem you describe. There is a small bearing around the front axle shaft that turns as the shaft turns, which occurs when the hub is locked or when in 4wd or both. Other than that one bearing & the attached axle shaft everything that turns in 4wd or with the hub locked also turns in 2wd or with the hub unlocked. If your problem also occurs in 2wd I would first suspect brake drag, probably from a MC or booster not fully releasing. If it positively only occurs in 4wd it's something in the 4wd driveline. If it only presents after extended driving I would look for a bad bearing or a dry differential, but either of these should produce a noticeable noise. Drive the truck at speed in 4wd with the hubs unlocked, listen for unusual noises, see if the problem recurs (you can do this on dry pavement since the front tires are freewheeling). IF the problem arises feel for overly hot areas among the various front drivetrain parts, from the wheel hubs in to the differential & back to include the transfer case. Report back your findings.
_________________ '78 Chevy LUV,1.9,4sp,headers,31's,SAS Dana 30,Dana 20,SOA rear w/Aussie Locker, rough body, bought new 12/4/78. '87 Trooper,2.3,5sp,headers,31's,ball joint flip & spacer,Aussie Locker rear,Superwinch hubs,brush guard w/5.5K winch,more to come.
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