The motor/bower combo that came with the truck was specifically sized for the additional air flow required with an air conditioning system.
Sufficient heating capacity can be achieved with much less air flow through it's heat exchanger (the heater core) than is required by the A/C system to cool the same space.
This is due to the disparity in temperature differentials (of the media flowing through the heat exchangers of the two systems).
In the warmest setting of the heat/defrost mode, engine coolant at an average of about 175 F (across the heater core) is used to raise the temperature of the air coming out of the various cabin vents. The change in temperature is from ambient (lets say 40 F) to about 100 F. A 60 degree difference.
With the A/C system operating at the coldest setting, refrigerant at an average temp of about 35 F (across the evaporator coil) is used to lower the temperature of the air going to the vents from ambient (let's say 85 F) to approximately 45 F. That's about a 40 degree difference.
So while the change in the temperature to the vent supply air, as it flows through each system's heat exchanger, is fairly close (60 F in heat / 40 F in cool) - the temperature differential between the heating/cooling media and the air supply is huge.
In the heater core of the heating system, a virtually unlimited supply of 175 F coolant is available to raise the cabin air supply by 60 F.
On the other hand, in the evaporator coil of the cooling system, a finite amount of 35 F refrigerant (limited by the capacity of the refrigerant system components) is available to drop the cabin air supply by 60 F.
So, as you can see, the heating system - with it's unlimited supply of much hotter water than the air being heated - can get by with a much lower air flow rate through it's heat exchanger. This is because the coolant is so much hotter than the air.
The cooling system doesn't have a huge supply of refrigerant available to it's heat exchanger. And, what it also does not have, is that big temperature differential that exists in the heater core. To compensate for these limitations the evaporator coil is sized somewhat larger than the heater core.
But the primary modification that had to be made when the A/C systems were designed for these trucks was in the flow rating of the blower motor/fan assembly. It needs to be much bigger.
Having said all that, the answer to your questions are:
Is there only a blower motor in a/c trucks? No,heat only trucks also have a blower and motor to drive it. Both the fan cage and the motor are smaller than the AC version.
Is there a separate one for heat and a/c? Yes they have different capacities and physical dimensions.
Or do I just need an a/c specific blower motor because my truck used to have the a/c?
You cannot use the lower capacity blower (even though you no longer have an A/C system) for the simple reason that it wont fit. I don't think it will anyway - though to be honest, I have never tried put a heat only fan in a A/C housing.
The AC blower cages are quite a bit bigger (length & OD) but the shaft diameters are the same, so I suppose you could try mounting an AC cage on a heater motor shaft and hope that the fan doesn't overload the windings. As far as whether the mounting flange on a heat blower can be mounted on an AC housing...IDK.
