Knuckle oil seal. http://WWW.myisuzuparts.com lists pn as 8-94407-725-1 for a 10-1991 to 01-1995 Trooper, and pn 8-97123-691-0 for a 02-1995 to 05-1996 Trooper. If you want an Isuzu oem seal, I'd send a pm to JLemond. Double check my part numbers just to be safe if you order one someplace other then Jerry, Jerry won't need no stinking part number, they're in his head.
Since you'll have this apart again anyway, at least check the knuckle bearing as well. Or better yet, buy one with your seal, they aren't that much and you'll have it on hand if you need it...likely from your description.