I'm not sure what size posts your grill comes with, but I bought these for my Genesis of the same time period:
(51) 7/16" Grip Ring Stem. Side Lock Brake. Load Capacity 440# Per Set.
www.ebay.com
1-1/4" Square Tube Socket for 7/16" Grip-Ring Casters, Fits 1-1/4", 18 Gauge Tube. Will fit the following Grip-Ring sizes 7/16" x 1-3/8".
www.ebay.com
Measure your posts - if they are 1.25 inches on a side then these inserts and casters will fit. I am pretty happy with them.