When a crystal goes, you should not get any response, however there is one thing you can try before you plop down money. Crystals are similiar to a lightbulb inside. The crystal is connected inside to two wirres and is suspended inside the case....just like the filament in a lightbulb. They can sometimes breaks away if subjected to a shock. TV/VCR etc remotes often stop working due to a broken crystal if dropped on a hard floor.

What you do is remove the receiver form the vehicle, but don't disconnect any wiring. Turn on the TX and Rx and operate teh Tx controls while moving the Rx in different orientations. Sometimes if you hold it jsut the right way, the system will start working again as the crystal makes temporary contact. If this happens, you kknow for sure you need crystals. If it doesn't respond, you still may need crystals, but it's a bit of a test.

I'm a little cheap, and I'd try everyting I could to narrow down what's at fault before I spent any money. As this problem started while driving in a field, I'd look at the steering servo and crystals, (as you mentioned) first. Sometimes a steering servo can just get bound up and puts such a drain on the battery, the voltage drops and even the throttle servo won't work. So try disconnecting the electrical connector for the steering servo and see if the throttle servo starts working.

If you still have the old receiver, you can try it. Even with the antenna off, you should get response at close range.