07 M8 rear suspension not holding any weight

have the rear shock rebuilt and put the proper size spring for the weight your carrying.

Do you have the fox air shocks? If so moisture may have entered the air chamber. You may need to rebuild the shock if so

fox shock water logged

as mentioned the most common issue with that vintage fox shock is after several thousand miles or less, water is shoved past the shaft seal ( the more often the shock is fully compressed or suspension full bottomed) and the water in the shock freezes outside in the cold and the suspension will stick or stock down. Then back in a warm garage the same suspension will feel ok.

Front shock rear suspension is the first to go...........needs rebuilt, at least oil and gas replaced. Rear of rear likely needs gas and oil also.
Many of the hair pin cat springs will fit the m8, we used king cat springs in 07 when nothing else was available. Try just one King cat spring, that might be stiff enough, usually was. You a heavyweight, try both sides. You will have to move the rail spring mount to make the shorter stiffer king cat springs work. Good luck.