It sounds like you've run MANY miles with that rocker arm out of adjustment. The constant pounding of the pushrod on the sloppy rocker has worn both of them to the point they finally disconnected. Valve adjustment at every tune-up is just a fact of life with solid lifters, and even hydraulics need the occasional check and readjustment. You might have been able to remove/install the pushrod without altering the cab if you'd removed the rocker shaft assembly first. I'd reccomend a thorough inspection of all the pushrod/rocker relationships, and replacement of any parts that are showing wear. Then, adjust the valves every time you change spark plugs.
Jerry
