Problem with VSS?

I have a 97 explorer
When I activate the cruise control it surges
It also intermitently grabs at highway speeds like it wants to kick into 4X4
Ford dealer said it was the cruise servo and that one tire was low(The dealer here is very bad)
Replaced servo still same problem
Someone else told me it was a problem with the VSS
This problem appeared overnight
Any info appreciated

Update
I have replaced both the cruise servo and the VSS and the problem is still there
Surges when cruise on and intermitentally "bumps or grabs" at high speeds with or without the cruise on :banghead:

I finally diagnosed it for those that have a similar problem
Replaced the VSS drive gear ($25.00) all is well