Shaking in 2nd 3rd and 5th

[Chrome_Rush]

I'm pretty sure this is a Tranny issue, but want some help. 1991 4.0 ohv 5 speed (m5od-r1) 2wd. Replaced clutch, flywheel and entire hydraulic system a year ago. Original tranny shifted smooth as butter, and still did even after this issue began a few months ago.

Clutch is smooth as can be, shifting is fine. I have shaking starting midway through second, worse in 3rd, goes away completely in 4th, and comes back in 5th. Frequency of vibration changes with rpm, sometimes it's worse then other times. It goes away when the clutch is pressed. It goes away if you take load off of it. It's vibrating through the chassis, vibrates the dash, mirrors, etc.

Rear wheels roll smooth, rear axles spin easily with no noise as does the rear end, which has no excess slop. Driveshaft seems tight at the u joints, which are newer, and spins with zero wobble. I would think any of those things being an issue the issue would still be there in 4th though.

I was positive I had failing bearings inside the tranny, swapped it with a used tranny, same issue, figured I possibly got another bad one, swapped in yet another one, still the same issue, but could SWEAR this 3rd one doesn't do it as badly. Is this a common issue, I've been hitting Google hard and can't come up with much on it. I can't think of anywhere else the issue could be given 4th is like new, and 2nd 3rd and 5th are vibrating the whole truck, even at cruising speed, the more load on it the worse it gets. It does seem to ramp up and get worse around 3k, and gets much less severe with high rpm

 

[fr7]

My 98 has a vibration but it's only in 3rd gear, and it doesn't shake as bad as you describe, I can see and feel the shifter shaking. Also sometimes it tends to grind going into 3rd, like when you don't have the clutch all the way down and try to shift, does yours do that too or not? I replaced the tranny fluid 3 days ago and it seemed to help with the grinding a litte bit. I believe mine is a synchronizer, yours might have something to do with that but I doubt itd shake the truck that bad, could be bearings like you're thinking..

 

[Chrome_Rush]

Your issue sounds like 3rd gear might actually have some damage to the gear itself, a synchro wouldn't cause a vibration once in gear, but a hard or grinding shift. My syncs are all good, and I pulled the top cover on both replacement transmissions and inspected and found no visible issues. That doesn't mean the bearings couldn't be on their way out in them all though. I'm just looking for other ideas before I try yet another transmission.