More than about 1/16" vertical movement at the end of the shaft on the driver's side is pushing the limit. The greater the play, the more air that draws uncontrolled around the shaft. At some point, the throttle plates no longer index properly or close completely and/or the plates stick and/or you can experience poor idle control and erratic TPS voltage. The shaft can be bushed, but great care must be taken not to twist the throttle shaft screws off in the shaft. Look down the bores at the throttle plates with the plates closed. If there are gaps between the throttle bores and the edges of the plates near the axis of the throttle shaft (or the throttle shaft can move axially with the plates closed) then the plates are excessively worn and rebushing is a waste. If the truck spent years on a farm, dust may well have trashed the base, shaft and plates. LPG and propane can have similar effects, because they are "dry" fuels and deprive the throttle shaft and plates of any of the lubricating properties of "wet" fuels. Good luck.
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