98GT w/4EAT won't shift out of first

[BMG905]

My neice has a 98 GT she got from me, it now has 204K on the clock and the tranny won't shift into second or above.

 

It does engage reverse and first.

 

Engagement into reverse is quick and firm, into first less so.

 

Local shop tried a drain and fill with filter change and it worked for 2 days, now back to the above state.

 

Anybody have similar experiences?

 

Any not need to replace the tranny?

 

Can the valve body be removed and cleaned to remedy this?

 

Any tests that can be done without disassembly?

 

Thanks for any help.