Had this problem a long time ago, I've helped a few members to fix it over the years as well.
Looks like some people ordered the entire assembly, but it might not be needed as I fixed it with the parts inside my RS2.
Depends on how handy you are. Basically what happens is the small cog that runs up and down on a toothed track wears out.
However, in my case, and I suspect in other cases as well, only few of the cogs' teeth wear out, leaving the rest of them intact.
When you activate the manual vertical shift it works until the missing teeth align with the toothed track, skip..., the lens drops.
Same mechanism is used in the horizontal plane, except it does not lift the actual lens assembly which weighs about a pound or so,
but rather shifts it left or right, no gravity, no wear.
What I did is used the horizontal cog as a donor for my vertical cog. Switched them around. Then I aligned the broken cog in the horizontal plane in such a way that the broken teeth were at the extreme of the range and did not affect my horizontal shift.
No parts, no ordering, no cost, just a bit of tweaking. Works like new.
Best way to go about it is to take the cover off and have a good look and see how the whole thing works.
After that it's pretty simple to understand what I explained above.
Hopefully this helps