An axially-floating sub tool for axial load compensation in conjunction with a top drive comprises cylindrical upper and lower members, with the upper member being coaxially disposed within a cylindrical housing. The upper member has an upper section slidably disposed within an opening in the upper end of the housing, plus a middle section that slidably and sealingly engages the housing bore. The lower-member bore has a splined upper interval, and a coaxial stinger extending upward from an annular shoulder medially located in the lower-member bore. The lower member is connected to the lower end of the housing with the stinger slidingly and sealingly disposed within the upper-member bore, and with the lower-member splines slidingly engaging the upper-member splines, thus defining upper, middle, and lower annular chambers, with the middle and lower chambers in fluid communication, and with a regulator/check valve regulating pressure in the middle and lower chambers.