solenoid pack

We can engage reverse only after warm up and driving at least 10 minutes and then it is still weak backing up. Is a solenoid pack hard to change? Can I do it myself? Any ideas? Is chaning the fluid and filter sound resonable to do and expect the reserve problem to go away. Seems odd that the reverse gear would engage (even slightly)after warming up), makes me think it is a fluid/filter, solenoid, etc... problem. Thanks. Dodge Grand Caravan FWD 2001.