Could use some help...

I have a 1999 S-10 ZR2. Recently my service engine light has been coming on, And..When it changes gears it hits VERY hard....and wont shift into overdrive. I was talking to a buddy and he said the same thing happened to him. He took it in and was told it was the valve body. Could this be a do it yourself fix? I havent messed with auto transmissions before, Never even changed the fluid in one. Is this a very big job to do myself??

Thanks

you can do it your self if you have the complete transmission teardown book and have someone who has done it to help you, if not take it to a tranny shop