I have no doubt that an X5 35d can haul a 5,000 lb. rig, even with several hundred more pounds of load (gas, boating equipment, etc.). But I wouldn't recommend doing this on a regular basis for long runs at freeway speeds or mountains. IMO the issue is the automatic transmission, which is reported to be a fairly common weak point on X5s.

FWIW I'm one of only a handful of X5 (E53 and E70) owners on this forum who have posted that they fairly regularly haul near the upper limit of the X5's range i.e. an all up weight X5 + Trailer + load of 11,000 lbs. But I only do this on relatively short hauls e.g. typically less than an hour and never more than a few hours, one way.

Funf Dreisig