Make sure the transmission fiuld level is correct; being sure to follow the manufacturer's instructions about what gear to be in and whether the engine is running when checking the fluid.
If the fluid level is correct; the transmission may be failing. Chrysler automatics have had a poor service record when used behind the Cummins diesel; they do not seem to be able to handle the Cummins' torque. Dodge came out with an optional Allison heavy duty automatic transmission in later years; which is expensive; but solved the transmission failure problems.
If the transmission is trashed; you could have it modified for heavy duty use by a shop that builds racing transmissions; or adapt an Allison transmission to this vehicle. In either case; this will not be cheap.