sorry to hear about that mono -- I've had a sore throat for a little over 3 weeks now, and it's really pissing me off. I can only imagine what you're going through.
As far as the enlarged spleen goes... your spleen is in your upper left abdomen. The concern with mono is that external pressure on the enlarged spleen will cause it to rupture. While this is a very real danger for mono cases -- it is extremely rare. So, providing that you FEEL strong enough to hit the gym -- you can do it, with caution. Be aware of the symptoms of a ruptured/bleeding spleen though.
Pain in the left upper part of your abdomen (under the left chest), feeling lightheaded, feeling like your heart is beating fast and hard, bleeding more easily than usual and having trouble breathing.
If any of these symptoms occur, I would stop the workouts. I realize a lot of the symptoms are pretty much what it feels like after a heavy set -- so, I wouldn't go nearly as intense as normal.
Be cautious and patient. One of the things about mono is the weakness -- don't go all out in the gym.... just go to try to keep some strength.