From what I've researched, no one wore number 21 for the San Diego Chargers in 1998.
Junior Seau!
Ryan Leaf.
George Roberts
RB Glen Bonner.Bonner wore #21 for the Chargers in 1974 and 1975.
Chad Rinehart is number 78 on the San Diego Chargers.
Ryan Matthews
Dwight Freeney is number 93 on the San Diego Chargers.
John Phillips is number 83 on the San Diego Chargers.
Johnnie Troutman is number 63 on the San Diego Chargers.
Keenan Allen is number 13 on the San Diego Chargers.
Kendall Reyes is number 91 on the San Diego Chargers.
King Dunlap is number 77 on the San Diego Chargers.