Yeah i just dealt with this, started shuddering and i couldn't find anything wrong... long story short I probably did another 300 or so miles after the shudder started and it grenaded my whole fuckin' tranny.
The one-way clutch outer housing cracked, causing the rollers to jam. The shuddering you feel is the rollers being un-stuck when the second gear clutch catches. Basically the first and second gears are working against each other because the one-way clutch is stuck locked up.
Eventually the cracks will get so bad that the rollers will just straight up jam solid, it will tear up or break the clutches or eventually put so much force into the first gear gearset that the teeth will break and get chewed off. Also it makes the reverse thread nut (center of 1-way bearing) tighten itself on insane-o tight.
In order to get the motor apart i had to get that nut completely glowing red hot with an oxy torch 3 times, hitting it with an impact wrench. Destroying/ruining the temper on the countershaft in the process.
Pretty much the whole transmission is gone from me ignoring the shudder!