I'm way over here in Oz, where my problem would be the opposite, ie plants probably frying in a summer move. but since no-one has rushed in to help, I thought I'd write.
First off, is your new home still within the Illinois state boundaries ? , as there may be transport issues across borders.
2nd I guess, is how long is the trip likely to take in hours, and can the transport company guarantee a straight trip without lengthy stop-overs?
3rd given that plants are "perishable" in these conditions, they really should be last things on the truck and first things off. But, without sitting around somewhere while the packers do other things. Are you expecting to be there at departure and getting ahead of the truck to be there at the other end when they arrive?
4- have the removalists any prior experience with moving live plants? Over here there are a very few specialist plant transport companies, but it's certainly not cheap - and in your case you have the temperature issues.
5- I guess you can prepare a bit, and that will depend on the size of the plants etc. like packing tops of pots with stuff to keep the soil in if they tip over, or preferably putting pots into boxes to make them easier to move, and wrapping trunks and leaves to minimise damage. The pots themselves would need to go into plastic bags to stop moisture leaking out and weakening the boxes.
6 - all this preparation, takes up space, and space with removalists is definitely money. I guess you need to consider whether in fact it is economical to move the plants at all, or whether you have other options like selling them and buying/propagating new ones at your new home. (I know, that's probably not what you want to hear).
7 You might well fit a few plants into your SUV, even large ones if prepared well and placed on their sides, but you can't build them up to such a height that they impede your vision when you are driving, and as you say it's a long return trip just for plants.
8. So I guess I'd start making a list of absolute must-haves, and others. Work out a square foot area of the must-haves based on truck floor area and see whether it's economical to take them. Save boxes. bags, and newspaper and whatever wrap you can find. Things like old sheeting may protect a bit from the cold during the move. Get a proper reception area ready for the plants at the arrival point so they don't sit outdoors freezing.
9. Utilize your friends and plant people to "mind" stuff that you can't fit in, in case you can come back for more SUV trips, or if other people are travelling in your new direction and can afford the space to take a few plants to you.
10. Not being really used to the cold that you probably experience, but I wonder if there are products that you can spray on the plants that minimise the impact of the cold, and consider individual plant needs as to whether it is safe to trim things back a bit over the move without setting them back too far.

wishing you a happy move, and not too much stress!

Hi alisonoz,

We are actually moving ourselves. Do you think bare rooting the plants would work? That way, I could fit more into the warm car.

Four years ago we had to move on the coldest day of the year with our citrus trees. What worked for us was wrapping the trunks with strips of soft fabric and putting a plastic bag around the canopy. We may have wrapped the pots in blankets as well. They all made it. Best of luck!

Four years ago we had to move on the coldest day of the year with our citrus trees. What worked for us was wrapping the trunks with strips of soft fabric and putting a plastic bag around the canopy. We may have wrapped the pots in blankets as well. They all made it. Best of luck!

curlygirl- you moved them in an un-heated vehicle? How far?