GMC Repair: 1999 GMC Automatic Transmission, costly repair, manufacturer recommendations

Question
I have a 1999 GMC 1/2 ton 4WD truck with an automatic transmission and electronic shift 4WD.  

My transmission sometimes has problems shifting from 3rd to overdrive.  It seems to shift fine until it warms up.  The problem was intermittent, but has now become a norm.  When the transmission heats up, it will stay in third and sometimes try to shift into O/D lockup.  It may also try to jump back and forth.  

The transmission is not slipping.  It drives fine when you select 3rd from the column.  No prior repair.  All service has been done per manufacturer recommendations.  Bought the vehicle new.

Can you help me with this problem.  I believe it is a solenoid or sensor, but the dealer is not sure.  

Thanks in advance.
William

Answer
It sounds like the overdrive cluthces are slipping, I would bring it to your local dealer and have it checked out. It is a costly repair if I am correct but a diagnosis should not be too bad. Also if you decide to get a reman unit from a dealership it comes with a 3yr 50,000mi warranty.

It may be a sensor or solenoid, but usually that would cause the check engine light to come on.