Since you are new, I have to ask this.
Are you using 4wd on a hard surface where it should not be needed? If so, that is your problem.
I don't know squat about automatic transmissions and their tcases, except yes the tcase has a chain not gears. I also know tcase problems are rare on toyota trucks unless they have been run dry or otherwise abused.
Do check for driveshaft rubbing - the dust shield on the front driveshaft CV is not hard to bend...