This gear stucking is quiet common in wrangler, never heard in Grand Cherokee. You have to seek professional help for sure. Its not DIY stuff.
Car needs to be lifted, gear needs to open and then see if its repairable and how much damage is done. All it could be gear lever have small rubber boot, it may be damage and now stuck coz of sand went in that lever boot. So you got to change that. For that also you need professional help.