If it's nearly new, you're in luck diff-wise: A diff that is too loose can ruin the balls, plates and main gear in no time. I recommend you rebuild it; Do you have previous experience with ball diffs?
I once built an SRX2 and found some parts needed special attention:
- The plastic shock cartridge (in which the o-rings are) can be a little tight around the shock shaft. Ensure that the shock shaft moves through it freely.
- Ensure the shock piston moves freely through the shock body - a bit of sanding (or scraping with a scalpel) might be needed, but you do need to retain the smallest possible gap between piston and shock body.
- Take care inserting the diff screw into the outdrive (because of the o-ring) - greasing it helps.
- The diff's T-nut is a plastic piece moulded around a metal nut (if I remember correctly) - it can strip if it is tightened too far (brutely). As such, check its condition during the rebuild. If it is stripped, I recommend buying the Exotek's T-nut for the Kyosho RB6; it also fits the SRX2 diff.
- Take the shocks off and check all suspension arms fall down under their own weight (virtually friction free). If not, you might need to sand or scrape a little material off the wishbones.
A free suspension and well built diff are corner stones of a good service life and good pace with any car, so its worthwhile checking it out
PS #1: If it turns out you are missing spacers, I recommend going to your local hardware store. They should have 6x3x0.5mm steel washers that carry out exactly the same function for only a fraction of the money
PS #2: I just saw your note on the tight gearbox. If it's just somewhat tight but still moves when you turn it, it might need to be run in. Before you do, do check the condition of the bearings and see to it that no parts are obviously catching or binding.