a 1998 should have a drain plug underneath. Simply drain the fluid and refill to the right level, about 8oz maybe a little more I think. You should really take the cover off and give it a good cleaning, check all the bearings, sprockets and drive chain. Look for wear and replace parts if needed. Had a bearing fail on me this winter in the middle of the woods because I didn't check it before the season stated. :-o
To remove the cover drain the chaincase fluid. remove side body cover. remove exhaust. (you don't have to but it's alot easier if you do.) remove reverse lever pin, and two bolts that hold the reverse cable in place. Remove 4 bolts around the case cover and center bolt that holds the reverse gears in place. Try and salvage the seal if possible but deff replace if torn or worn at all. Clean and inspect everyting, roll the sled if possible to see if there is any play with the bearings, Upper (Brake) and Lower (Driver shaft). Clean again and replace. Check chain tensioner as well, make sure to tighten to 20 INCH pounds. (Finger tight) and tighten lock nut. Fill with 8oz of polaris chaincase oil and check for leaks.