But I'm confused as to why the pair of diagonal braces is bad? Are you thinking that the joint would weaken the 2x4? I've built trailer walls (OK, more like a fence) like this - and they hold up over the road pretty well.
"bad" is a poor term to use. The diagonal braces are a good solution for what's presently done, but the way I'm picturing it, you're introducing two angled cuts into the mix that will need to be measured and cut well in order for them to fit. I'll post a crude picture next to show how I'm interpreting it. Throwing in a half-lap introduces even MORE woodworking into an equation that is already not satisfactory to the person who produced the initial project. Even without the half-lap, it will still require angled cuts that may or may not solve the problem.
My college statics classes tell me that the x-brace in the back would stop the side to side wobble pretty well - the front, perhaps not, but I'm hoping theres a front installed on his cab to stiffen it up. With no front stiffener, there'd be some kind of bending moment set up if you rocked that half of the cab, but thats another story all together.
It's the best (and cheapest) solution. College or not, that's THE quick and dirty fix right now. Without introducing any angles into it, I was just thinking throwing two more horizontal braces in there in the back (because I've gotta believe there's a front too, otherwise that accounts for ALL of his slop

) is gonna stiffen it enough to satisfy.
Hmmm, sounds like different advice from your 7 step list earlier ('just throw a couple 2x4s under the monitor shelf'). And why are diagonal braces a 'recipe for disaster'?
My bad on being vague. I'd split them up, one prolly above the monitor, one below, as most folks are already paranoid about their monitor falling (who could blame ya's since it's prolly one of the more expensive parts and a pain to replace!) Not an addition to the current monitor shelf, something OTHER than the monitor shelf.
The monitor shelf itself is already helping to stiffen your cab and keep it from feeling even wobblier (usually - each monitor shelf is different, your mileage may vary) if you've got a "square" frame to set your monitor on, meaning a square made of wood which would fasten to the sides AND the front and back. That frame is helping to somewhat square your shell.
Generally, with objects this tall, flex comes from a top corner to a bottom opposite corner, which is why a back to something like this is THE best way to solve it. The cross braces are the NEXT best, and simple horizontal braces are the least best. Notice that in the order from worst to least best, they also get progressively more involved. That is what I meant to imply about a recipe for disaster. Please, anyone who might think I'm telling you the idea is a waste of time - I didn't intend for it to sound like that. I just think either make the back, and do it right, or if you're worried about a slight wobble, throw in the horizontals. If it doesn't solve it, you're out $6 and some saw-blade wearage, and you'll want to add a back to solve the problem for good.
(whew, trying to shorten answers was recommended. I just am not the guy to give it a try.....EVER

)