I used a bivvy for years when backpacking and did the same on a couple of offroading trips. The bivvy works great when backpacking as nobody else is around. When offroading, I find that it's nice to have a little privacy to change, clean up, etc. It's also nice to have a little extra room to move around, and a tent is definately more comfortable in the rain. With that in mind I'd recommend the smallest tent you can comfortably fit into.

In my mind I just wanted a tent that was essentially a bivvy. So I went for a single wall 4-season mountaineering tent. With no rainfly it is really easy to setup. If it isn't wet, I just shove the whole thing, with my sleeping bag inside into the back seat of the truck. Stash the poles, and go. Makes setting it up the next night really easy.

TNF, Mtn Hardware, Bibler, etc all make some pretty nice mountaineering tents.

Craig

I agree with Craig that the smallest tent is the way to go. While I like the 4 season tents, they can be hot as hades in the summer time. I have a mid 90's Walrus tent that is more of a conventional 3 season backpacking tent. You might want to possibly look into a 3 season tent that has more than 2 poles to provide some extra support if you get snowed on. On the Whipsaw run a few years ago, mine got a few inches on it and wasn't sagging at all. The nice thing about a 3 season is that if there is no rain in the forecast, you can use it without the rainfly and get a nice view of the stars.

Either way, it just depends on what types of conditions that you are planning on camping in most frequently. Each are a compromise in some regard.

Mikes right. A single wall 4 season tent can be very warm in summer, and it can also collect a lot of condensation. Not all of the 4 season tents are created equal. A good double wall tent works year round.

A really good double wall 4 season tent is the Mountain Hardware Trango series. I had one for several years before I broke a pole and wound up returning it to REI. I replaced it with a single wall tent because it is smaller (warmer in winter), and easier to setup. The single wall isn't nearly as comfortable as the Trango was though. The Trango breathes fantastic even with the rainfly on, and is absolutely bombproof. It's become one of those legendary pieces of mountaineering gear like the MSR XGKII. It works year round and is comfortable year round.

I usually let my gear (tools, shovels, etc) sleep outside under the truck, and I sleep inside the 4runner with the food and beer. That'll change when the pickup gets up to speed, so will be watching the suggestions.
When I was young I bed rolled it under a tarp tied to the headache rack

been happy with my half dome from REI