[MDimitri]

Correct me if I'm wrong but the 2003's came with the 4L60E transmission, and the 2005 models started to come out with the 4L80E, if you had a choice take the 4L80E. But in regards to your initial question I'm pretty sure they use the same transmission. The cost seems a bit high to me, what are they giving you for the core exchange? Shop around a bit more if your really concerned about the price, there's alot of place's you can get these transmissions from.

4L80E Description: Comes Standard on Chevy Silverado 2500 4x4 6.0 liter 4-speed automatic transmission.

Transmission General Description:
The 4L80-E is a fully automatic rear wheel drive electronically controlled transmission. The 4L80-E provides four forward ranges including overdrive and reverse. A gear type of oil pump controls shift points. The PCM and the Pressure Control (PC) solenoid (force motor) regulate these shift points. The PCM also controls shift schedules and TCC apply rates. Transmission temperature also influences shift schedules and TCC apply rates.

You can operate the transmission in any one of the following seven modes:
P - PARK position prevents the vehicle from rolling either forward or backward on vehicles less than 15,000 G.V.W. For safety reasons, use the parking brake in addition to the park position.
R- REVERSE allows the vehicle to be operated in a rearward direction.
N - NEUTRAL allows the engine to be started and operated while driving the vehicle. If necessary, you may select this position in order to restart the engine with the vehicle moving.
OD - OVERDRIVE is used for all normal driving conditions. Overdrive provides four gear ratios plus a converter clutch operation. Depress the accelerator in order to downshift for safe passing.
D - DRIVE position is used for city traffic, and hilly terrain. Drive provides three gear ranges. Depress the accelerator in order to downshift.
2 - Manual SECOND provides acceleration and engine braking or greater traction from a stop. When you choose manual SECOND, the vehicle will start out in first gear and upshift to second gear. You may select this gear at a vehicle speed of up to 22 km/h (35 mph) .
1 - Manual LOW provides maximum engine braking. You may select this gear at a vehicle speed of up to 13 km/h (20 mph)

Good Luck KenDog.