Acceleration Problems and Vibrations

I have two problems.  I'm having trouble with acceleration when the outside temp hit around 80 degree F.  If I give it too much gas the engine revs., but slow acceleration as if the Catalytic Converter was clogged.  I just removed the Catalytic Converter and straight piped the exhaust system.  At first I though it was the converter, but this problem still exist before and after the converter was removed.  Also, the service engine light was on and stated: EVAP Leak and O2 Sensor, so I was advised to replace the gas cap and O2 and I did but still proble exist.  I just had work done on my transmission at the dealer, so I don't think its the trans; however, I'm getting vibration when I hit speeds above 50mph.  I put the trans in nuetral to see if I had a balance problem and no sign of problems.  Could this be my U joints?  Maybe its time to get another vehicle.
 
2001 Chevy Blazer
4.3L Vortec
Mileage: 210,000

This definitely sounds like a transmission issue.  If the engine just revs without a corresponding increase in speed (assuming a gear change or TC unlock did not happen), then the transmission is immediately suspect.  A plugged cat would show signs of resisting higher RPM loads.

What were the exact codes (Pxxxx) that were present?

Vibrations can be caused by numerous things.  Tim (hanr3) created a thread awhile back on the causes of vibrations - Vibrations

Let us know on the codes.

The codes were P0420 - Catalyst Efficieny Low Bank 1 and P0446 - EVAP purge vent solenoid circuit condition