The description is a little confusing. Are you saying that the pulley
rocks at an angle in and out like it's broken at the base or that the whole assembly slides straight in and out on the end of the crank shaft?

It's not likely that the bolt that holds the pulley to the end of the shaft has come loose but it's worth checking. Reads like something is loose.
Certainly explains why you're chewing through belts. I don't know if there are thrust bearings in that engine but 1/2" of play sounds excessive for that scenario.

'89 P'up, 2.6 I-Tec, 488,000 miles and done... gone to the great beyond