High inpute gear & shift select gear
Had the same problem, under any torque my brute would pop out of high range. Reverse & low were fine.****** transmission select gear & high input gear were destroyed. From engine mounts being bad, engine will tweak under high torque, linkage will stay stationary due to notches in selector knob plastic & kick it out of gear. Gears will first weaken & bevel the edges @ a 45 degree angle (supposed to be square to assure lock-in) then will only get worse due to it being worn out & keep popping out of gear untill they are completely destroyed. Recommend fabricating engine mounts &/or replacing them & im 99% sure u will need those gears... Hopefully just the fwd high gear if u catch it in time. (you will need to disassemble engine & if you done do it soon you can scatter trans & crack case $$$$$$$$$$$$!!!!!) hopefully i help!!! :d