I'm here in Central IL as well (Gardner). I supply heavy equipment components and we specialize in articulated trucks and wheel loaders. I can tell you that we supply probably more Volvo Transmissions than anything else (typically 9000-13000 hours is the life of the transmission in these).
More than likely its a point to the transmission, but I can't tell you how many times I have sold a guy a transmission to find out he has a bad harness or an ECU Problem, and its a costly mistake.
We have a computer box we use on our dyno to shift the transmissions we rebuild, I could loan it to you to help diagnose... what you can do is hook it up directly to the transmission and try to shift the truck while sitting in the cab (or anywhere else using my shift box. If the truck shifts with my box, you know you have an electrical issue (ECU or harness). Hopefully that is the issue for your sake of course.
The issue with some of the dealer software we find is everything points to the transmission itself. The other day I was working on a L120E With a "bad transmission" they brought into our shop, it would not function in reverse, that needed a $130 sensor to fix the whole issue.
If you need some help or some copies of a service manual, you can email me at email@example.com. I would be happy to help.