It is call an enclosed driveshaft - a torque-tube rear end.

A 1954 Truck Shop Manual does not seem to be readily available but a 48-51 manual (with a 52 supplement) is available from many vendors and on eBay (see manual-link below). There is one universal joint in this set-up.

http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm