Unless you have a 2.7 3rz equipped 3rd gen you do not have a timing chain. All 3.4 trucks have a timing belt that is recomended to be changed every 90k, most swap the water pump while the motor is opened up. If you do have a 2.7 and a timing chain then your mechanic is probably trying to get you tp spend money, the chains should easily last to 150k and up. If it starts to rattle around and makes lots of noise then it may be time to change the chain, if not leave it alone. Either way the 2.7 and 3.4 are non interfernce motors, so if the belt or chain breaks you will simply be stranded, not in need of a top end rebuild.
And originally Toyota did recomend 60k on the 3.4, they switched to 90k around 98 or so, it really doesn't have anything to do with it being a 4 or 6.