3-4 shift rail guide mushrooming has been becoming more of a problem as of late. If you install your tranny and it feels like it gets stuck in 4th gear or the shifter feels sticky when going into 4th you may have a damaged (mushroomed) 3-4 rail guide in your case end cover.
The pic below shows a part of the case that sticks out and houses the 3-4 shift rail. Sometimes, when the person working on the vehicle is in a hurry or trying to take too many shortcuts, the transmission will get stuck between the drivers side wheel well and case end cover. To get it out, they will stick a prybar in between the transmission case and the drivers side wheel well and force the transmission out. This will compress the area where you see the arrow. The 3-4 rail slides in and out of the case in that spot. If it is mushroomed, the rail will get stuck and no longer move freely. The same applies during install.
If a prybar is used to force the transmission in, you will damage the case. Loosening or removing the subframe assembly under the steering rack will make a lot more room for easy removal and install of your trans even if installing with a twin or triple disk clutch assembly. It looks to be a lot more work removing the subframe, but it is well worth it as everything will slide in so much easier and parts are a lot less likely to be damaged. If this has happened to you, we can repair the case.