yes. it will last a long time. i have replaced steering boxes before and i am now pretty convinced that they werent worn out but my mechanic had turned the screw the wrong way to adjust it. if you can get the looseness mostly out it will last for thousands of miles more.
dont know about the diff except to say they are extremely long lived too. if not run low on grease. i only know of one that wore out and it was run a long time with two different sized rear tires and boiled the grease.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.