Many times, as the drums wear, they will develop a lip on the inner-most portion where the shoe doesn't make contact. If the shoes are adjusting properly through their lives, this lip may be enough to prevent easily pulling the drum off.
The solution is to dial back the star adjuster, accessed through the lower part of the backing plate, just enough to allow the drum to come off. Keep count of how much each is backed off so you can dial them back in pretty close on re-assembly - or put up with more brake pedal travel for a while until the auto-adjust feature dials them back in.
If there's much of a lip, may be worth considering having the drums turned while they're off.
If you're going to this much effort on a dry run - why not complete the job? Once the vehicle is jacked up, wheels are off, and the drums removed - not that much more to do the seals.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 217K miles. Repainted Oct09.