Speedometer won't register above 17 mph
 

I recently bought a 2002 f150 4.2, manual trans and the speedo just won't rise above 17 - 20 mph. All other gauges in the cluster work ok. The odometer seems to work, but not correctly because of the speedo. Took it to speedo repair shop and was told that it is probably ok, but problem might be speed sensor. Also, ABS light and check engine light stay on. Is all this related? Cat has been removed and a pipe welded in. Clues?