What he is looking for is a MOBILE antenna, I use a couple that work quite well. The Antenex/Laird ABSCAN and the Larsen NMO-150/450/800. Both work well.
If a system is out of sync it will be a bit of a problem to overcome. I know I'm going to get people saying it doesn't happen, but it happens all the time.
You may want to check your roam settings, if you are talking about an 800 trunking system, you might want to check your AGC setting as well. Lots of this is quite experimental.
And yes, even a cheap ks pretty darned good for the money, I've done it before.