* Bolt patterns: The transmission's bellhousing, which connects to the engine, will almost certainly have a different bolt pattern.
* Output shaft: The output shaft, which connects to the transfer case (if applicable) or driveshaft, will be different in size and/or spline count.
* Computer compatibility: The transmission's control system (whether electronic or mechanical) may not be compatible with the 1500's engine control module (ECM).
* Physical size and mounting points: The 2500 transmission is likely larger and heavier, and its mounting points may not align with those of the 1500.
Even if you could overcome the mechanical differences (which would require significant custom fabrication and machining), the mismatch in torque capacities could lead to premature transmission failure. The 2500 transmission is built to handle significantly more torque than the 1500's engine can produce, leading to potential issues.
Instead of attempting this swap, you should focus on finding a suitable replacement transmission for your 2000 Ram 1500. A used or rebuilt transmission specifically designed for that model year and engine is the most practical and reliable solution.
