Welcome to AF!
It's either the transmission or the shifting mechanism.
There are about 9 things that could keep it from staying in gear. 5 of these are related to the internals of the tranny. They are:
1. Worn shifter fork.
2. Worn synchronizer hub or broken sychronizer spring.
3. Worn synchronizer sleeve.
4. Excessive thrust clearance.
5. Worn bearings.
If the tranny shop put in a rebuilt these things probably aren't the problem. If they they rebuilt the tranny themselves, I'd say they have a problem.
The way they put the tranny and shift linkage back in could cause it to jump out of gear if they didn't do it right. If the shift linkage was damaged or the gear shift lever support shaft spring was weak it could cause it to not stay in gear. Also, if the gear shift lever socket bushing is worn it could keep it from staying in gear.
No matter how you slice the cake, they did the work, they put it in and they should stand behind their work. Saying it isn't this or that is just putting you off. Sadly, there is no way you can prove that the synchronizers are bad unless you have it torn down and inspected. Stay after them. Since you say you had it replaced I assume you had a new or rebuilt put in. Take it back and have them check all the linkage and the rear tranny mount. If you had a rebuilt or new tranny put in there should be a warranty on it. If it was a used from the boneyard they should still make good on it for at least 90 days. Remember. The squeaky wheel gets the grease. Be a PITA! If they say it's not this or that, say back to them, Well, what is it?