AWD disable light on

'04 Pontiac Aztec AWD, 6 cyl, 36,000 miles.
About 20 miles into a trip AWD disable warning light came on. Pulled to the side of the road and it went out, but came on again a few miles later and has remained on. Tires are original and pressures OK.
Mechanic diagnosed 'clutch pump check valve'.
Just wanting to make sure there are no other possibilities before I spend $500 for replacement.
Thank you, A Kurt