Any idea as to what was causing the problem?

 

Actually this might be possible:check the caliper on the RH! It is a given,if its not working-the unit will pull left because thats the only front brake that is working(the LH unit in your case)Look see & let us know!

 

i just finished going threw this problem with my 98 blazer, i replaced all of the steering arms idler tie rods bearings, ended up finding out the upper control arm bushings were non existent on my right side and left side only had 1 bushing, so i had both sides have new bushings pressed on and aligned it now its strait after 2 alignments, o also my steering gearbox had a bunch of play in it.

so check to see if you have any upper or lower control arm bushings on both sides.

 

Most alinment shops today only perform a toe alignment. Many cars today do not allow for caster and camber adjustments and many that can be adjusted require tabs to be knocked out in order to perform these adjustments, which just happens to be something most shops will refuse to do because they're just to lazy or their computers tell them that a caster and camber adjustment isn't required. I agree with some of the earlier post's, first check the brakes, then the suspention front and rear, if you don't already know- google alignment so you're more informed as to what needs to be done then find a new shop, make sure they will do a complete alignment (caster and camber as well as toe), inform them of your problem, tell them that you want them to drive the truck in a real world test and not around their parking lot. Sad thing is that many shops want to turn as many cars as posible and to many times they cut corners and don't do what they should to begin with.
I bought a set of tires for my 98 GMC from Pepboys, 33's that required lifting the front a bit. This throws off the caster and camber. I had the front end lifted prior to going in and it wasn't untill the tires where on the truck that I found out they wouldn't do the caster and camber as part of the alinment. After close to $900 in tires, I was pissed and refused to pay for the alignment. I took my truck to a refered shop, after the caster and camber were set he did the toe alignment with a tape measure and string. It works, I drove my truck for years without any abnormale ware, even with the big tires.
Good luck, and remember if you can't steer straight your running in circles!!!!