1991 240 wagon shakes violently at 1,110 RPM

[Svenska]

I'm hoping someone has experienced this. I just did a simple timing belt replacement on my friend's 1991 240 wagon. I also did the four-fan belts and the bushings for the p/s pump and alternator. No problems with any of this. I drove the car and did not feel anything unusual. My friend drove it for the first time today. He told me the car shakes (steering wheel & driver's seat) violently at 1,100 RPM and then the shake goes away higher than 1,100 and is not present lower than 1,100 RPM. He did a test where he puts the car in DRIVE, applies the foot brake, gently increases the RPMs to 1,100 and it shakes. It does NOT shake in Park or Neutral at any RPM range, just in DRIVE, either with his foot on the brake or driving at that specific RPM. Obviously it has something to do with the transmission (I'm guessing). He brought it to a local garage, they inspected the carrier bearing, motor mounts and (if the car has one) the transmission mount. Everything is intact, and the mounts are only four-years old. It's almost as if the driveshaft has suddenly gone out of balance, although it was never removed. Any ideas?