I think you need to show them examples of what you like in a logo. Designers are not mind readers and I think you need to find a way to express what you want. Spend some time and go through atleast 5 at most 10 character logos and print them out and show I like X on this because of this. If you are unhappy with your local designers there are sites you can go to see other designer's work and pursue hiring them.
I don't want to tell them what I want as I am not a designer.
To this comment I think you need to figure out what to tell them and if ALL the designers, as you have mentioned, provide you with the exact same thing. You should look at changing how you deliver your requirements, intention, desire, thinking. You clearly have something in your head of what you want and you got that from somewhere so try to think of where. Also, try to draw it out, not kidding, a sketch of something is better than nothing.
Just to clarify, since this is one sided, that there is a graph I would like you to understand when looking for character designers or any designer at that:
After the edit:
I want a logo that really jumps out at you. I am much less interested
in brand identity etc. I just want it to be memorable. It also has to
be the centerpiece around which I will "wrap" my trucks.
Are you giving them this information of what type the vehicles are?
This is an important factor and is something I try to stress to every person that is wanting a vehicle wrap. Each vehicle should be treated as its own design and that is why I am curious to know if you are telling them this or just providing a size area and saying here.
If these are for wraps have you considered hiring a wrap company instead of designers?
Wrap companies tend to employ their own design team that can address the issues you are facing and possibly provide better results compared to designers who do not understand how to design on vehicles.
Every designer I have received samples from just doesn't seem to
deliver on what I'm looking for. They all do variants of the same and
none of them seem to hear me and take big risks. I don't want to tell
them what I want as I am not a designer.
How are you in returning feedback? Are you bouncing off one designer after another and fishing?
I ask this because some designers will provide a starting point and want to know where you are wanting to go with it.
How can I encourage them to really push the boundaries and produce a
colorful, humorous, memorable logo? What can I say or do?
What are you delivering to them? Are you providing feedback of what you like? Are you providing a color palette?
You should narrow down an area.. Just allowing a designer with an open canvas, in this case, will produce results you are not looking for. Figure out a palette, you say memorable consider looking for a character designer. You need to explain you intention or goals. You have to have a goal in mind or you will keep bouncing around.
What is your low and high points for your budget on this?
Not trying to be rude.. but the fact of the matter is if you only have a budget, lets say of a couple hundred, you are pretty much on point with the quality. No one, that I know of professionally, would give you heart, sole, creativeness, and a low budget design.. Also, it would appear you are asking for samples from the amount of bouncing you have mentioned which would cause you to get a vast amount of work that is similar and not edgy.