The Extension Housing Bushing
The extension housing is fastened to the tail of a transmission or transfer case. There’s a gasket in between the two machined surfaces. If the extension housing has small nicks and burrs on its mounting surface, they can be removed with a file. If it's warped beyond specifications or too badly damaged, it must be replaced. Always check with manufacturer's specifications before making a decision. The mounting surface can be checked with a straightedge and a feeler gauge. The housing contains the output shaft and most often the vehicle speed sensor. The slip yoke extension housing bushing is pressed fit into the housing, and then the extension housing seal is installed to contain the transmission’s fluid. The extension housing is sometimes called the tail housing.
A worn extension housing bushing can cause driveshaft vibrations. This is often caused by an inherent lack of lubrication. They also wear on one side of the bushing. A binding u-joint or misalignment will certainly cause this accelerated wear. Check for sagging mounts or anything that can cause transmission misalignment. Move the slip yoke up and down, there should be little play. Check the slip yoke for pitting; pitting will prematurely wear the new bushing resulting in a comeback.
These bushings may contain an oil hole that must match up with the oil supply passage in the housing. This is true of many bushings in an automatic transmission. Never deprive a spinning shaft of its lubrication. Bushings are made of brass or soft metal so they wear before the slip yolk.