2004 Ram 2500 Tranny problem...HELP!!!

My 04 2500 Ram 4x4 Quad Cab won't shift .Automatic tranny. It takes off in 3rd never using first second or overdrive, just drive. Reverse works fine. Will not manually shift to 1st or 2nd. Took it to transmission shop,they ran diagnostic that said the computer is telling the tranny to shift but it won't. They want several days and a lot of money just to dianose the problem.
I said no thanks to that.
Any help is much appreciated.

IS the speedometer working.

IS the speedometer working.Yes the speedometer is working

I'm not sure exactly how it works, but most modern day transmissions have a "limp home" mode that does exactly what you are describing. The trans goes into this mode when it detects a problem that needs attention. My 96 Chev truck did the same thing. The dealer replaced the ignition switch to fix it. Go figure.. A trip to your dealer (or another trans shop) may be a better choice than where you went.

Dodge 545rfe trans:

"The hydraulic control system design (without electronic
assist) provides the transmission with PARK,
REVERSE, NEUTRAL, SECOND, and THIRD gears,
based solely on driver shift lever selection. This
design allows the vehicle to be driven (in “limp-in”
mode) in the event of a electronic control system failure,
or a situation that the Transmission Control
Module (TCM) recognizes as potentially damaging to
the transmission."