I have a HPI MT and used to have the exact same problem.
The best solution I have found are these:
Stick one or 2 (depending on how much play there is) in the differential cup. Should solve ALL you problems. I like them much better than the spring idea because they are kinda "slippery" and absorb a lot of vibration.
A funny thing is this quote from tower's site.
"These are the replacement outdrives for the Rush
Located inside the front differential"
Last time I checked the Rush didn't have a front diff.