Transmission not shifting from 3rd to 4th sometimes

[JEG in Raleigh]

Hello.  I have a 2006 GMC Sierra, 6 cylinder, automatic transmission with 175,000 miles.  I bought it used with 165,000 miles on it to use as an around-the-house work truck.  I only put a few thousand miles per year on it.  Anyway, the transmission went out on it a couple of years ago and I had it rebuilt by a local shop with a good reputation, that I had used in the past for other trucks I had when I had a business.  The truck runs well enough and serves my limited purpose, but in the last year, it has started something new with the transmission.  If I accellerate a bit hard at all, it will stick in 3rd gear and not upshift into 4th.  To fix this, I was having to come to a stop, put it in park, but it back in gear, and take off again at my usual leisurely rate of accelleration, and it would upshift as it should.  I also recently discovered that when this happens I can put it in neutral while coasting at driving speed, and put it back in gear, and it will fix the problem.  But it is quite consistent that it will stick in 3rd if I accellerate a little bit hard at all (and this thing does not accellerate very quickly, LOL).  I don't want to spend another $3500 on a transmission.  I'm wondering if something is sticking maybe and if I should try Sea Foam?  I have not had the fluid changed since the rebuild, but I've only put about 5000 miles on it since the rebuild. Do I need to change the fluid?  There is no warranty. Any advice would be appreciated. I'm a lousy mechanic and usually screw up things worse if I try to fix it myself, so whatever I do, I'll need to have a mechanic do it. Thanks. 

[txab]

No magic in a can. Why not go back to the shop and let them look at it? Especially since you state this:

 

Quote

I'm a lousy mechanic and usually screw up things worse if I try to fix it myself, so whatever I do, I'll need to have a mechanic do it.

 

[richard wysong]

could be the 3/4 solenoid but  you need to take it to a qualified shop to diagnose the electronic transmission unless you are prepared to buy a scan tool that does live data then take it there anyway for the repair. sounds like you have a good relationship with the rebuilder, I would at least go back for a diagnosis, I doubt it needs another rebuild