yup, its the same raquet you see these guys selling the sx57's and l55's l60s on trucks under cdl.. the truck weighs 24k out of the factory and then is overweight after you put your welder and bottles on. check out the Crane center in ohio, there website might not be up to date but they are one of the best in the business with skyhoist/manitex cranes. call and see what they have in stock. Not sure of your budget but you could probably pick up a great condition tandem 87 for around 100-125k. a single axle 87 in decent shape go for 60-80 as you may have noticed and there is a reason. most have been flogged.
Are you replacing that 80' knuckleboom truck you have? isnt that thing a tandem? a sx57 and rx87 are a big jump from one another. which one do you see yourself using more? I forget what other trucks do you have to support this on an install?