If there's nothing loose in your xfer case that's jamming things, could your parking brake be engaging randomly due to loose shoes or too-tight shoes? Could there be an unrelated problem in your rear axle that coincidentally showed up at this time, such as a loose pinion nut?
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner