Fullarton, 26, has clocked up just over 50 appearances since joining Palace on a free transfer from French club Bastia three years ago, but was told by Alan Smith last month that he did not figure in his plans.
The former Scottish Under-21 international (above) began his career at St Mirren and is likely to jump at the chance to return North of the Border.
Meanwhile, Jackson met with United boss Alex Smith at Tannadice on Thursday to negotiate a move and if they agree personal terms the Scottish international will join straight-away.
United sought permission from Hearts about speaking to Jackson, now 34, who has has been told he can go because the financial restrictions at Tynecastle triggered a dispute over a clause in his contract which stated he was entitle to an extra year if he passes the 15-game mark for the Jambos this season.
Jackson - who spent for years at Tannadice before moving to Hibs in 1992 - has already played 14 games this term and Hearts have told him he will sit in the stand for the rest of the campaign unless he agreed to waive the clause.
A Hearts spokesman said: "Dundee United asked if they could speak to Darren and we gave them permission to do so."
Unied chief Smith is desperately seeking a cutting edge to lift his team, who have netted just seven SPL goals, off the bottom of the table and Jackson has always provided goals wherever he has gone. He was top scorer with Hibs in 1996-97 before his move to Celtic.
