If you NEVER want to use the XM on your bike, then yeah, buy a seperate unit for your truck. You can find XM recievers for $60.00. Compare that to the $170 for the Garmin XM unit. If you just want GPS for the bike and also don't want the Bluetooth options, then you will save even more money getting the Zumo 450.
The subscription price for XM is the same regardless of unit. The traffic/weather feature is nice, but adds about $4.50 a month to that. I think they do sell some stand alone XM radios that use this feature, but not sure.
If you get the Garmin XM unit you will also have to find a way to get the XM to play through your trucks sound system. You will have to have either a stereo input in the trucks radio or a cassette tape converter (if it plays cassettes) or you can buy a FM transmitter to send the signal to your trucks FM radio. (You will still need one of these to listen to any MP3's you put on your Zumo too)
The XM Roady2 and XT has a built in transmitter, so if you just want tunes for your truck, this is way to go. As mentioned , the XM interface on the Zumo is not as handy as a stand alone XM radio car unit.
KM