sudden immobility of dog

Question
My dog Hank, is a terrier/cur/fiest mix. We thought he was going to be a small dog, but he's got some LONG legs!! He stays in the house, but the last couple of days have been so pretty he's spent alot of time outdoors. Last night, before we went to bed, Hank was himself- normal and playing. Jumping around like normal, and up and down off the couch as normal. This morning however, he wouldn't get up and walk. He had gotten in a clothes basket (I'm not sure if he'd been there all night or not) and got out when I woke up, but wouldn't walk or put any weight on his back legs. He got back in the basket but was shaking very badly. I came home at lunch to check on him. He met me at the door and walked out with out putting any weight on his  back right leg, and very little on his left. Once he made it to the porch, he just fell over and stayed there. (he's not whining, or even acting like he's in much pain...) I had to pick him up and take him back in the house where I made him a little pallet and put him on it. That was 2 hrs ago. I just went back and checked on him and he's still in the same spot that i put him and didn't even try to get up when I came in. He's still shaking really bad- mostly his back end and 2 back legs. I love my dogs as if they are my kids, but our financial situation does not allow us to take them to the vet without saving and planning. Any help you could give me will be greatly appreciated. I don't want to lose him, and don't want him to suffer in pain.... I just don't know what to do.
Oh- and we've always called hank our "neurotic" dog. He has some very peculiar behaviors, and he's VERY hyper normally. So to see him almost lethargic really scares me. Could someone have poisoned him yesterday while he was out? Could he have gotten hurt by another animal or automobile? I would think if this were the case he would've shown some kinda symptoms last night..... But he was normal as can be last night, but then this morning he's just basically completely immobile.
Thanks for listening. Once again, I'll be grateful for any advice you can give me.

Answer
Sudden inability to use the legs coupled with shaking and lethargy usually equals a dog that has hurt his back somewhere, usually around the sacrum. That is the triangular piece of bone where the pelvic connects to the spine. It can also be a fractured pelvis, or a spinal injury. Shaking is a sign of pain in dogs. Dogs won't whine as much because in their head that is a sign of weakness and the weak get eaten...

He may get better on his own, but there is no way to know without a hands on exam of this dog. A small buffered baby aspirin will help with the pain, but pain and shaking and inability to use the back legs will get worse and less treatable as time goes on if there is something seriously wrong. Again, a hands on exam is the only way you will know.

If the aspirin helps, he may have just pulled the muscles around that area. Do not give him any other over the counter pain medicines.
He is looking to you for relief. As responsible pet owners we have to provide it.You might want to look into getting pet insurance on him.

I hope he gets well soon. Please let me know what you find out is wrong with him.