It's been more than a couple of years since the last time I had my column apart, but as I recall, to replace the ignition lock (tumbler) you only need to get past the turn signal switch and (possibly) the steering lock plate, which requires a special tool (oddly enough, called "steering lock plate removal tool" at most discount parts stores). Possibly someone can verify whether I remember correctly on the steering lock plate.
One thing to note: It sounds as if you are saying the tumbler turns and starts the truck without the need for the key. If so, no problem - just replace the ignition lock as you stated. If, however, the lock is turning (with or without the key) but is not activating the start switch at the base of the column, you probably have a broken ignition rack, which does require going deeper into the column. The part can still be found at the discounts for under $20, and you'll want to search the phrase "Inside the GM tilt steering column." It's a 69 page pdf with exceptionally helpful photos. Very step-by-step. Good luck !