I was afraid of this, but I'm glad that there are other people here that can confirm my problem. I'll have to scour my work area for it, or figure out what else would make an acceptable substitute for a shim. I Honestly thought I could get away without it.
Say, do you think computer paper/inkjet photo paper would make a good substitute for a metal shim? Also, cramej, did your LM have four shims all around? I only noticed two in my camera, which leads me to the conclusion (among other things) that someone mucked around on the inside long before I got it. (Unless manufacturing processes weren't all that good then, and Yashica only used as many shims as it took to balance the focusing field).
I actually ran a roll of film through it two days ago, just shooting whatever's around the house (outside). Seemed like everything was working in order, I have noticed that every wind gets progressively shorter, but I assume that's due to the takeup spool getting progressively wider. It would be interesting to see what I'd get, once Christmas is out of the way. Considering that I shot it on tri-x at f/8 - f/16, I don't think I'd see anything weird.
Though, this morning I was playing around with the camera (unloaded), and I was composing a shot of my window, and I noticed a shift. I also noticed that the viewing lens exhibits pincussion distortion. Could anyone confirm that this is true, or just a symptom of misalignment? Thanks for the replies.