What aero mods can i make to a 1999 jeep grand cherokee?
 

My budget is $500 but im open to all suggestions.

Do you have any goals? Any limitations? Do you off-road? Is this strictly fer pavement use? Do you use the rear hatch? Which model/trim level do you have? Have you tried any mods yet? Cardboard works well fer experimentation... ;)

Post up actual piccies of yer Jeep... :D

Grill block would be a start to find out how much you can block and still stay safe fer cooling. Use a digital meat thermometer to monitor under the hood temps along with an Ultragauge to monitor yer actual coolant temps. Belly pan would work. I made the one on my truck very modular and in 3 sections.
Under belly pan... - Toyota Nation Forum : Toyota Car and Truck Forums

Here's a mesh grille...so a block would be possible from behind the grill... ;)
1999 Jeep Grand Cherokee Mesh Grille N/A

Here's what someone did to their Grand Cherokee Laredo... :D
http://media.motortopia.com/files/66...Napoli_038.jpg

mods
 

Most of your money is spent creating an enormous turbulent wake.
Any kind of boat-tail extension would pay you big dividends.
It could be built onto a receiver-hitch type cargo platform.
It could be an inflated structure attached to a swing-away spare-tire holder,inflated with any 12-VDC ventilation fan.
They've worked for me,they've worked for NASA.
It's probably the single best mod you could do.:)