In basic terms, the purpose of the driveshaft – also called a propeller shaft or prop shaft – is to transfer torque and rotation from the transmission to the axle.
Slip yokes are common where 1-piece driveshafts are installed in vehicles that have a sprung rear axle (as opposed to a fixed location IRS). The tube of the yoke is splined on the inside and it engages splines on the transmission output shaft.
As the rear suspension moves up and down, the driveshaft will plunge towards and away from the transmission. The slip yoke literally slips in and out of the transmission during the driveshaft’s plunging motions.
TREMEC transmissions for classic cars, muscle cars and other vehicles use a slip yoke to connect the driveshaft to the transmission. Fixed flanges or fixed yokes are normal in systems that have fixed differential mounting and independent rear suspensions. This includes the Gen 5 and Gen 6 Camaro and the 2008 and newer Dodge Challenger.
There is no movement in the rear axle, so the driveshaft doesn’t need to move up or down, and the distance between the output flange on the transmission and input flange on the rear axle remains constant.
Instead of a slip yoke, a fixed flange or yoke is bolted to the output shaft of the transmission and the driveshaft is coupled to the flange through universal or constant velocity joints.