[rocky_mtn_man]

My '91 V2500 4WD Suburban has a one piece rear driveshaft. With 4WD, the rear shaft is shorter so there is no need for a longer shaft with carrier bearing. This 4WD drivetrain is much smoother than my older RWD '74 Suburban, where I had to replace the carrier bearing and take the shafts to a drivetrain shop for professional balancing and to replace the u-joints.

I had a hammering vibration develop under no load at 45mph recently and inspected both driveshafts; when researching online I found an answer to a worry I had about the front driveshaft. It is splined so that it can move forward and backward when turning or going over bumps, and there is a grease fitting which I lubed, along with all the other front end parts accepting grease.

I was investigating why when you don't have 4WD engaged the front driveshaft can rotate back and forth about 1/16" of a revolution. Jeep and 4-runner forums said this is completely normal and don't worry about it. If there is any up and down slop in the differential yoke or the transfer case then you have a problem.

My vibration problem turned out to be old worn out tires that were letting go all at once, internal belts were separating and banging on them with a wood block and sledgehammer to free them from the hub (due to rust) probably didn't help matters. I had Big O tires put it on a lift to make sure the drivetrain was ok, it was only $65 for a half hour diagnostic check to rule out any drivetrain problems. I was on the right track by starting to swap in another tire set I had and the problem improved with the first tire I put on.

Thought I would reply to this thread, aptly named, and mention the small, expected rotation of the front driveshaft when 4WD is not engaged, in case anyone searching these boards is wondering about that.