The lock washer shouldn't matter, but if the thrust washer between the sleeve and the bearing is misssing, then it may. That thrust washer was not used on the pre-unit boxes, they had a slightly longer sleeve. I never have figured why the washer was added, I can't see why the spring acting directly against the bearing inner was a problem!
Good luck on this one, keep us posted,
On the pre-units the sleeve butted against the bearing inner radius which provides little surface to support the clamping force from the nut. That's the reasoning behind the thrust washer, only it's to soft to be really effective. On most gearboxes I work on the nut will be loose due to that. In my experience one of the things they never really got sorted. No biggie though, I use Loctite on the nut and torque it to 25-30 lbs-foot. I never bother with the lock washer, It's a waste of time.
On the problem at hand, the 4 speed box relies on the camplate plunger to keep the gears engaged, and the dogs are straight cut. Faults to look out for are worn engagement dogs, worn cam-tracks, worn rollers, worn shift forks, a weak plunger spring, and so on. In a word: look for wear.
Worn and tapered engagement dogs will exert axial forces on the shifter forks that can overcome the plunger spring. It's a very real problem on many high mileage machines, and replacing parts is the only real option for most of us.