Why won't my Ram 1500 Van shift into Reverse?
 

OK, so I had to park my full-size Ram Van with the Tusscanny conversion package when gas got over $2.00 a gallon. As a result, I haven't really driven it much in the past 6 months. I do crank it up every week or so, and move it forward or back a few feet to keep the tires in good shape.

Yesterday, the wife's PT Cruiser got a flat, and I didn't have time to take it to get a new tire, so I decided that she'd take my car, and I'd just drive the van.

So the Van starts right up, but after warming up won't shift from park to reverse.

I was running late, so I didn't have time to pop the hood, just drove the Wife's PT with the "doughnut" spare.

Any thoughts on what this might be?