This doesn't sound like a power steering issue to me. I can't imagine the power steering being the root cause of the issue of "darting"/wondering as you describe. This is a bushing issue and/or trac bar and drag link, and/or other front end components.
As described it sounds like now that you have [good] power steering and this allows the root problem to fully manifest itself or exacerbate the *real* problem. In other words, now w/ power steering you feel the sensation better than you have previously.
Re-reading your description and the part of you just installing new C bushing, Caster! Are you 100% sure that you installed the *correct* degree bushings and 100% certain you installed them in the correct orientation?
The C bushing are labeled inside with writing such as "lower", or "upper", ect. Some bushing mfg have different indentification, but they all have some stamp so the installer can get the correct orientation. If these bushings were installed "upside down" they wouldn't be the first ones........
Think of Caster like bicycle forks when we were kids.The front forks are raked forward. You can ride the bike easily and sometimes take your hands off of the handle bars and the bike will glide along in a straight line. As you ride the bike it always goes straight forward and doesn't require much steering input to stay on course. Now flip the handle bars around so the forks are raked backwards....now its difficult to control and requires constant steering input and still seems squirley. Many Bronco's don't have and never did have enough Caster and this is one of the root problems w/ driveability. ...the other major issues is the trac bar tying the front diff to the frame yanks the diff sideways everytime you hit a bump that upsets the suspension. I like to see 6 or 7 degree's of Caster. The toe-in alignment means nothing to the darting issue you're having.
I'd take this machine back to the local alignment shop and get a full print out of your actual Caster numbers. A Bronco needs 6 or 7 degree's of raked back [positive] Caster to drive nice. It wouldn't surprise me at all if you have 0 degree or raked forward [negative] Caster of 1 or 2 degree's
Here's a good link about Caster and darting/tracking/wondering
http://www.wildhorses4x4.com/product/Bronco_Wandering
Another side issue that comes in effect is the diameter of the steering wheel. If you're using a stock 17" -or so, steering wheel and used to sawing on it to get the machine to turn, and now have the sensation of a slight adjustment moves the machine sideways 4 feet, this is common. This "problem", which is actually a sensation and dangerous driving issue gets worse w/ a close ratio steering box and good power steering. One of the best cure's is an aftermarket 14" diameter steering wheel.