So Ive got a litte sonoma with the 4.3 and a nv3500.
My buddy bought it new in 01, and at one point he had a B&M short throw shifter in it. Later on he let his nephew use it to get to and from high school, and the nephew replaced the short throw with a stock shifter.

So now when it is cold outside the truck will shift into 4th and as long as it has throttle it will stay in 4th, but when you let off the throttle it will slide forward into neutral. It does not "feel" like it is shifting all the way into 4th when it is doing this.... Well today it did the same in second. Now once i drove about 15 or 20 miles and it got all the way warmed up it stopped.

What I want to believe is the nephew didnt get something perfectly lined up when installing the factory shifter. The problem was only noticed when by buddy got the truck back after the nephew drove it for a few years but that could mean anything..

this truck has spent the last 20 years at above Gm's recomended limits for basically everything

 

So it’s maybe 15 degrees this morning. This is the coldest I have ever driven the truck and when I went to start it I could barely move the shifter out of first to let it warm up. It feels like the shifter is full of frozen molasses

even after I forced it into neutral just sitting in neutral it barely moves side to side

I do know that it has the proper gm synchro mesh fluid in it

 

It could be the shifter mechanism, pull the boot, is there any adjustment? It also could be a worn synchronizer. How old is the fluid? A drain and refill is a good idea, it could be full of filings.

 

I’m gonna make some room and pull it inside in a few. I’m probably being too optimistic but I hope it is something basic. When it annoys me I just go 1-3-5. You can’t hardly lug it down but it annoys my wife too. So now it is on the list.

 

Maybe I got lucky. Took the shifter apart and found a 3/16 nylon spacer under the factory gasket. Called my buddy and he confirms that the nylon part was specific to the old b and m shifter. So my thoughts are that maybe the change in throw was causing my issues.

need to order a new rubber gasket as well

 

Wanted to wait until I drove it cold which I did this morning.

it did not pop out of gear one time. The shifts into 2nd and 4th were clean and crisp with a definate click into each gear

I have to get the big gasket ordered and then I’ll put the center console back in. As long as the center console doesn’t change anything I think it is fixed

 

Good to hear you were able to get rid of the annoyance without dropping the tranny!

 

Feel like I should get a powerball ticket