If the metal plates in the load cushions are exposed or there are marks on the buckets caused by contact with the "T" on th espring leaf it is time to replace your load cushions. This is not a difficult job but it does require some large tools. If the trunnion bushings (bushings on the suspension crossmember between the axles) are worn it gets much more complicated. Depending on the suspension rating the u-bolts will be either 1 1/8" or 1 1/4" for the 34,000-46,000 lb suspensions. These tighten down to 1,100 lbft for the 1 1/8" and 1,500 lbft for the 1 1/4". These must be tightened down or you will break spring leafs when loaded. There are three different trunnion bushing types: brass/greasable, one piece encapsulated rubber trunnion bushing, and two piece rubber trunnion bushing. If you have the brass bushing or the one piece bushing they will have to be pressed in. Also the one piece pushing can sieze to the dead axle making removal very difficult. When installing new insulators I would recommend installing urethane replacements as they seem to last much better than the current standard rubber type supplied by Mack. I will find a picture of the different trunnion bushing types so you can figure out what you have.