Transmission in 2nd Gear Default Mode, HELP!

Like the title says, 93 Astro Van, stays in second. I believe that the 93 is electronic controlled. So what sensors, etc. are common problems to check, and what should I do first? Any help is appreciated!!!

Probably should start by pulling codes from the ECM. Good info on this Camaro web site:

http://sethirdgen.org/dtc.htm