Hi there ,
Just to ask when your engine& gearbox was removed did they replace the e-gear potentiometer sensor?,its possible this sensor could be giving a false reading therefore you are having problems at times engaging reverse,but to get this done you would have to again remove the engine & gearbox assembly to do it ,also did they do allignment of your egear actuator after rebuild? If you are going to remove again the engine & gearbox assembly best you ask them to also check & replace your egear accumulator ,that is a black ball nearby your egear valves group ,this can only be replaced once engine & gearbox is out.
Although not common ,there has also been issues with egear wiring harnesses ,but this is a very rare occurence.
Regarding your front axle,if there was an issue with it once your on highway it should show on your dash the 4WD warning light,if this pops out it means front axle is overheating,another symptom of front axle overheating is oil will be thrown out from front axle breather ,if this doesn,t happen most probably your front axle is fine.
Wish you best of luck with your car,BR.