Tires are the most important component when considering grip under power. Second would be drive train layout, third suspension.
Simple advice: buy tires appropriate for the roads you will travel on.
Mostly off road? Buy off road tires. Mostly on road driving in summer months? Buy summer tires. Drive in winter? Buy winter tires.
Stay away from all-seasons, they're bad at everything.
I've always owned and driven RWD vehicles and used to teach RWD performance handling.
As an aside, if you're getting stuck on gravel roads and have appropriate tires then I'd look at one of two things: 1) Driving technique, spinning tires is bad, slow steady application of power to keep tires from spinning; and 2) an old van like that probably has an open diff, if you're really struggling an LSD would be faaaaaar cheaper than an AWD conversion and likely do the job just fine.