What exactly is happening? Is the "slip indicator" in the instrument cluster lighting up? Is the engine output being automatically reduced? Are brakes being automatically applied? Are you skidding?
What is the road surface of your country road like? Gravel? Recently oiled?
I assume that you have verified that your tire pressure is correct - 35 psi or a little more in all tires - but I would think that your TPMS warning light would be on if that is the problem assuming that you have a working TPM system.
Traction Control and Vehicle Stability Control work together. They are complex safety systems and not something that most amateur mechanics can repair when there is a problem. One of the causes of a TRAC problem can be a defective ABS sensor. At the heart of VSC is a module that includes a gyroscope - I assume it is in the floor at the center of the Sienna like on the Lexus vehicles I'm more familiar with.
As a temporary measure (after checking tire pressure), you could see what happens if you turn off TRAC or both TRAC and VSC. Press the TRAC off button on the dash quickly one time to turn off TRAC (the TRAC off indicator in the cluster will light up). Press and hold the TRAC off button for three seconds to turn off both TRAC and VSC - both the TRAC off and VSC off indicators in the cluster will light up.
If you can't figure out this quickly, I recommend that you take this problem to a Toyota dealer or a knowledgeable indie repair shop ASAP.
Is always good to provide the model year and trim (LE, XLE, etc.) of your vehicle although in this case the only trim that has a different TRAC/VSC/etc. system is the Limited with Advanced Technology Package.