I have a 91 Silv.w/ 4.3, manual tranny. The rear suspension is lacking to put it mildly. Is there any way to beef it up(stack springs) so it will handle more weight? Don't have much $$.

Add a set of soft ride add-a-leafs to the rear spring stack. It will raise the rear up around 2" but will settle down to around 1.5" in a few months "if you haul any kind of weight at all". This is about the cheapest way to go .
Dont get the short add-a-leafs. They will give you a VERY rough ride.

Try going to local salvage yard and look at some springs from a 2500 or 3500 they should be able to handle more load and lift it a little.

Thanks guys,
The 2500-3500 leafs, are they the same width as the 1500 series leafs? If so than I shoudl be able to get away with just adding on or two to each side, right?

Yes I think they are the same length and width, Just get a tape measure and look for someone with a 2500/3500 I 've never noticed a difference in width or length. Another option is a rear shackle flip kit, The rear shackles are hanging you can buy a flip kit that changes them to a compression shackle and it'll give about 4" of lift too. Not usre if you to lift since the front is a beeyatch to lift.

All you probably need to add is the overload leaf on the bottom. It is about 1/4 to 1/3 the length of the longest spring, and is about 2-4x thicker. You could also just swap out the entire leaf pack from a heavier truck.

http://www.suspensionconnection.com/cgi-bin/suscon/2510-250C.html - bolt-on springs. Not sure if those are over your $ limit, but I know they work good. They don't sacrifice ride quality much either.