Chevrolet Repair: Losing Overdrive, shadetree mechanic, hi tim

Question
I have a 1993 3500 Chevy dually, I use to pull a 37' travel trailer; lately when I pull the trailer within 10 minutes of pulling the trailer I lose overdrive and the transmission drops into drive and usually stays there no matter how much I reduce and/or increase my speed (it won't shift) until I stop and turn off the truck.  When I turn the truck back on and drive I get that gear back for a couple miles then I lose it again.  I have an automatic transmission and it only happens when I pull the trailer.  Is this a sensor and if it is where is it and can I a shadetree mechanic (parts replacer) replace it with moderate ease?   Thanks for your time spent with this question.
Tim

Answer
Hi Tim, There is a code in the transmission that will help with this problem. If you could find a place to scan it out It would help!! After you turn it off the module will reset and that is why you have OD again for a while. Could be a temp sensor or shift solenoid.

Hope this helps

Mark