My 06, 4WD, Diesel has 180K on it. I replaced the whole front end with a locking hubs aftermarket kit:
http://rvbprecision.com/diesel-stuff/dodge-cummins-diesel/ems-off-road-free-wheeling-kit-install.htmlAnd while I was at it I replaced the ball joints........other than that, the fan clutch went at 160K. No other issues.
I've done Nothing to the transmission. Changed fluid and flushed at 60K
When I bought the truck I put $4800 in a stock fund to buy an aftermarket transmission as I was sure this one wouldn't last. (I tow a 10,000# motorcycle racing trailer all over the place) Transmission is still working fine and my money has nearly doubled in 8 years!