4x4 problem

I have a 98 tahoe w/ 120,000 miles. Recently my service 4x4 light came on, and it stayed in 4hi. I took it in and they said it was stuck in 4x4. They unstuck it and cleared the code. A few miles down the road it came on again. I took it back in and now there sayin my 4x4 control module is messed up. Sayin its a $500 problem. Just wondering if any of yall have experienced this problem?????I would appreciate any help......thanx!!!!