A posi/suregrip/limited slip rear end still has the differential inside so that the rear wheels can turn at different speeds like they need to when going around a corner. Solid spools contain no differential, basically its a solid axle from tire to tire and makes going around corners a little tricky since the tires themselves must slip on the pavement. Talk to these guys, if your "posi" and diff. are good you only have to change the pinion and crown gear which come in a set.
http://www.richmondgear.com/07pdfs/RG8.pdf
