Wild guess on my part--I think your 'spacers' used to be a piece of wood that was bolted to the angle iron while also attached to the floor of the slide?? If the hydraulic ram moves the angle iron in/out, the board that ended up looking like a spacer moved with the iron, and since it was attached to the floor of the slide, the slide would move.
Any sign of where such a piece of wood would have been fixed to the floor of the slide? Any staples seen comeing down from the floor area?
Best I can think of...
By the way, ever find your old truck?
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died