To minimise disruption, cost and labour, I'd suggest you fit a new pair of suspension units, with nice new hubs, bearings, etc. Something like these:
http://cgi.ebay.co.uk/Trailer-Suspen...item5636caf939
If these won't fit to your existing channel axle, then you could buy/make something like this to do the job:
http://cgi.ebay.co.uk/Boat-Trailer-P...item3a549658ab
Rogue Wave's axle is very good value, but do you need brakes? Moreover, you may have difficulty fixing it to your existing trailer chassis, on account of where the fixing brackets will be (on the outer part of the axle, rather than near the centre)