Transmission Clunk

Doing a little towing the other day, my 1960 SII '88 developed a loud "Clunking " from the direction of the gearbox/front diff area. It first started whilst labouring up a steep hill, is not so bad with gentle throttle, but gets worse with more throttle, and is present in all gears.
Anyone got an idea, or where I should check first before I go to a garage? Thanks.

I have a check for you. Make sure it is out of gear and get under get hold of the prop shaft and push up and down hard see if there is play if not try the same but sideways. May be the UJ's on the prop shaft which are very cheep to buy. £5 last time I bought one. Is it out of 4x4 and free wheeling hubs out ( SWITCHED OFF ). When I first got a landy I drove it round in 4x4 with the front wheels not locked so everytime I set off I got a nasty bang at the front as the propshaft turned the diff and the diff only as the wheels was not locked.