To check wheel movement, get a big block of wood or a proper stand to sit the truck chassis on with wheels clear of the ground surface. Those huge tyres create to much friction to allow the wheel to turn whilst truck is staionary. DO NOT fit heavier duty springs as they ones supplied are more than adequate fir the job which is to protect the servo from shock - heavier springs will lead to damaged gears, broken or burnt pc board or gear pins in the servo.
With regard to the Y lead, yes this is recommended in the instructions for all the heavy duty servos mounted in trucks, boats and aircraft especially when, as one would expect you use a 2 ch RX in the truck, a receievr which has "BEC" clearly printed on the top = Battery Eliminator Circuit". These usuall 2 but a number of 4ch RX incorporate a regulator to allow the use of the 6 & 7 HD power packs in electric cars in lieu of a separate RX pack - similar to ESC in E/flight. MAX volt out is usually 5V at about 1A.
For full detail on servos refer to
http://homepages.ihug.co.nz/~atong/ and scroll down to "Radio Systems, Accessories, Alterations and FAQ" then sub section =
"Servo & TX alterations, calculators, clonepacs, make an ESC or winch, FAQ."
see also
http://www.hitecrcd.com/Support/Faqs/Faq_Genservo_.htm
regards
Alan T.