I replaced the shot drive belt either last year or the year before. The tension is pretty good; I don't think it's slipping, and the spring tensioner pulley in the drive belt system operates freely.Always check to see the drive belt has the proper tension. I was given a JDLX178 that the owner was told by the service center that his hydrostatic transmission was shot and cheaper to buy a new lawn mower.
When I got it home I found the drive belt was loose and I adjusted the idler pulley. Been running well for five or more years. I did change the fluid though.
Here is a description of how a hydrostatic transmission functions one of my sons did a couple years back.