Exactly how is your system connected? Are you pulling return air from the return air grill of the HVAC system? If so what is the static pressure of the return side? What is the static pressure of the supply side. If it is constricted as you say these numbers should be high out of spec. If they are you need to open up the duct work to remove some of that restriction.

Minimum Duct Size
Round: 10"
Rectangular: 6" x 15"
Intake Air Duct should be 10% larger
Maximum Static Pressure: .4" Water Column
Velocity: 1400 FPM at the above mentioned duct sizes.

Attached a link for a duct fan, may work but it is a band-aid. The real fix is to open up the duct work. If your return air is not ducted I would get that hooked up also.

Here is a link that might be useful: Booster Fan

Yes cold air return is ducted. Basically I am pulling out of the existing cold air return right out of the bottom of my heat pump, and blowing right into the upper plenum of the heat pump, and baffled the back side so it does not create a loop.

I am unsure of static pressure - its existing duct work in the house... The heat pump heats the place well...

I have an inline duct fan for one of my runs....the others I dont have access to any more because our basement was remodeled.

When the heat pump runs, the curtains blow when it kicks on....when the pellet furnace runs, can barely see them moving... It only has an 800cfm blower whereas the heat pump is around 1300....

May have found the issue.. I am currently running in manual mode instead of thermostat mode. What a difference! I must have the wrong thermostat because I am at 74 and climbing a degree every 30-40 minutes. Its about 40deg outside right now...

Looks like this may work out afterall....although I still may go for more blower CFM to help distribute the heat better through some of the more restricted runs.