It's been close to 35 years since I last tore into the 'punkin between the front wheels on a jeep. Mine was a '58 Willys, so I don't even know if it's even comparable, but here's my take.
If you have the front hubs locked and in 2H, then the rotation of the tires (assuming under movement of the vehicle) is turning the gears inside the housing and also the shaft/slip joint. No, you're not getting driven power from the transfer case, because it's not in 4wd, but you're inducing turning in the gears and associated shafts from the front wheels.
T-case in 4wd, but hubs not locked in, again, you're turning the gears inside the housing and shafts, just not transferring that power to the wheels.
I'd be safe and inspect everything very closely. At a minimum you probably need a new u-joint, but if there's ANY wear on the collar or bolts, be safe and save yourself problems down the road. Change it all out, because it makes a helluva racket when that slider shaft leaves the vehicle at 40 mph and thus the front driveshaft starts dancing around on its own!