Navajo whines and vibrates.

So recently my 2-door Navajo started whining at high speed (50+mph) and and shudders at 15mph when slowing down. The whining and shuddering are not affected by braking, gear position, or clutch position. If I brake moderately hard, braking is smooth until 15-20mph, where the truck will shudder even if I coast down to that speed. If I nail the brakes, like in an emergency stop, I can make the truck shudder at any speed - making me think rotors, but again, brakes do not pulse or anything under normal braking.

This Friday I inspected front wheel bearings, rotors, and hubs, they are all good. I replaced pads anyways since it was apart. I then replaced the rear wheel bearings since I could get a slight tick out of them, and rear drums and rotors. While removing my axle shafts, I noticed that the rear driveshaft has a lot of resistance to it. I checked, tranny is in neutral. If I force the driveshaft to spin, it turns the engine over - tranny in neutral. So, I checked the tranny fluid - it was gear oil. I refilled it with mercon atf as it's an M5OD. The high speed whining is now reduced, and shuddering is reduced.

Now, I know I need a tranny, I've started looking for one. I'll replace it when I replace my clutch next year. I'll drive it as-is for now, it still goes through the gears fine, and shifts a LOT better with the atf in it. My question is, what is causing the vibrations? Is it really the tranny? Why can't I freely spin my driveshaft? Should I replace my radius arm bushings for the hell of it? They don't seem bad, they've been on the truck a while and have a little noticeable wear but are not worn yet. My next repair is tires, front shocks, and alignment. I'm going with treadwrights and monroe shocks (supposedly close to stock, and what I have on the rear now.)

u shouldnt be able to freely spin the driveshaft with the tires on the ground. could be the gears in the rear, if they are chipped or ate up it will whine and vibe on the coast side. when u checked the front bearings did u pull them out to check for scoring or did u just shake the wheel? does it make noises with it jacked up and spinning the tire by hand

I checked the bearings themselves, they are fine. I re-greased everything. I have also determined the whine is from the transmission - I am not concerned about that, I'm replacing it next year with an M5OD-R1HD. The truck was on a lift, I wasn't in my backyard, I spent 5 hours checking things and fixing things in my Uncles garage using his lift. When I took the diff cover off, the rearend gears all look new. Nothing funny came out in the fluid, I'm not concerned about the rearend. All I really want to know about is my vibration and if it's related to the transmission and it's above-mentioned issues.

Worn tie rod ends could cause the shuddering. Where do you feel it - through the steering? brake pedal? whole vehicle? Usually you can track down vibrations by where you feel them.

So if your truck is in the air, tires NOT touching the ground, and transmission in neutral you can't spin the rear driveshaft and/or it turns the engine?

Shran said:

So if your truck is in the air, tires NOT touching the ground, and transmission in neutral you can't spin the rear driveshaft and/or it turns the engine?

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That is correct. The motor turns over. Yet the truck shifts through the gears fine, and goes into neutral fine when stopped, truck doesn't creep forward or anything.

I feel the shuddering throughout the whole truck.

That doesn't make any sense. So given the scenario that I mentioned - and assuming your emergency brake is OFF - you should be able to spin the rear driveline and the rear wheels will turn but definitely not the engine.

What if you lift just one rear tire off the ground and spin it? How about both rear tires? Or if you have someone else hold the clutch pedal in, in neutral, tires off the ground, while you spin the driveline? Something does not add up here, there is a variable we are not seeing.

I didn't try it with the clutch in but the e-brake was definitely not a factor - my drums, pads, axles, and spiders were off. The tranny had the wrong oil like I said, and the whining and vibrating is now reduced with the atf in it - giving me strong evidence it is all linked.

Whining and vibrating are gone now that I've put a few more miles on the truck. It was definitely the fluid. Problem solved!