This is a modification I made to the front end of a Skag Zero Turn Riding Mower.
The caster stems were inserted up through a pipe welded through the square
tubing, and was worn to the point of binding, so the casters would simply slide
sideways across the lawn - tearing up the turf.  Modified by installing tapered
roller bearings on top and bottom of each stem.  Problem solved.
Caster1.jpg is the original 
Caster2.jpg shows the wear in the pipe bushing
Caster3.jpg shows machining off the square tubing to "flat"
Caster4.jpg shows a 2" pipe coupling, machined out to accept the race along
    with a short piece of 1-1/2" pipe used as a "race backup" to support the race.
    Probably not needed, but installed anyway.
Caster5.jpg shows coupling, support tube, and race installed and sitting on the
    flat portion of the square tubing (other "side" done in a similar manner).
Caster6.jpg shows the caster stem before modification.
Caster7.jpg shows threading the upper end of the caster stem to accept the
     adjustment nuts (one inch) and the remaining portion of the stem to be cut off.
Caster8.jpg shows the completed modification with bearings top and bottom, with
     locking nuts to maintain adjustment.  Also includes grease fittings top & bottom.
Thanks for looking.  
Ken.