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You mean the sprags wearing out faster? I would say they last longer, at least the front one, since there wouldn't be any slippage at all when the front wheels are "freewheeling". The sprag just stays engaged sending "power" to the front like you said, making the output and propeller shaft turn at 3049rpm while the rear spins at 3291rpm (2600/5th od. gear). And since normally both front hubs are disconnected it could even make one axle shaft alone turning up to 900rpm (!) if for some reason the other one offers great enough resistance.The sprague works by sensing the rear axles turning faster than the front. Once it gets to about 7% a clutch engauges and starts sending power to the front. With lock outs on the front, it will always try to put power to the front. So if you put lock outs on a sprague truck, it wears out the disks. Then you will have NO front wheel drive.
I concur with you and others, though: front lock out hubs on a sprag equipped truck just don't make sense at all; either convert to air shift and add two FW-hubs or start with just one hub/axle shaft kit for the center axle.
That is the best way to prevent rear (and front!) tire wear, less money invested and well worth it, IMHO.
G.