Hydro.tran. backs up slow an willn't back up small inclines the wheels don't spin you can hear the fluild by-pass in the trans. less then a 100 hrs on it.

The correct procedure is to get both wheels off the ground ( all four wheels if it is a 4 wheel drive ) run the engine at half speed, put the hydro in bypass ( push mode ) push lever ( or pedal ) slowly into forward motion and back to neutral, then into reverse and back to neutral, do this five to six times, now put the hydro back into drive and repeat the process, you should do this as many times as it takes to get good non judder movement of the wheels, if the machine has a separate reservoir then until there are no bubbles or air visible.

Jack the mower up at the rear and make it stable before doing any work on it, remove the rear wheels by removing the hub caps if they are still on the wheels and remove the cir clip and washer on each wheel and remove the wheels. there will be a key way on each axle when you remove the wheel, make sure they are put back in their place when assembling the wheels onto the axles or you will not have any drive on the wheels
Remove the drive belt, brake rod etc then support the trans axle before removing the retaining bolts and removing the trans axle from the mower. hope this helps.
If you are not mechanically inclined then you should leave the repairs of the trans axle to the mower repair shop unless you are buying a new or reconditioned trans axle, in which case you can fit the new trans axle in the reverse order of removal.

1ST THING YOU NEED TO DO IS CK TRANS FLUILD ON FRONT SIDE OF TRANS AND MAKE SURE IT IS NOT LOW,AND MAY ALSO START BEING SIGNS OF TRANS ISSUE,NEED TO HAVE CK BY TRANS SHOP IF FLUILD IS NOT LOW SO YOU WANT DO ANY MORE DAMAGE IF CASE OF TRANS ISSUE,HOPE I WAS A HELP

I don't think you can assess the condition of the transaxle by spinning the wheels.
The differential in the transaxle converts power from the transmission to the wheels by a set of gears. It would almost impossible for one side to turn at a different speed than the other.
You may have more drag on the brakes on one wheel, or the fluid in the transaxle could cause one side to go slower with the van off the ground.

with hydo trans you need to release the trans to push it around and they will spin like that

sounds like the trans is going bad but make sure it is full when it is running in park and on level ground

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the torque convertor is a mechancal part of the transmission. it converts rotational motion. it is located in the trans mission. it bolts to the fly wheel. not an easy job to do. messy also. put car on jack stands.un do battery. remove drive shafts.place support under transfer case. remove cross members. support trans mission. remove transfer case. support rear of engine. unbolt torque converotr from trans mission. remove transmission. pull torque convertor out. it took 8 hours to do this on our tahoe.

To me it sounds like the hydro unit has developed internal leaks and the pressure lowers as the oil thins from heat.These trans axles are usually replaced as an unit but some can actually be repaired.call the dealer to see if you can get parts or whether you need to replace the whole thing.I assume you have already tried a new drive belt,tensioner ect. and if so you are down to the trans axle.You may buy some time buy getting rid of the low viscosity oil in the rear end and go to regular hydraulic oil.

Check the drive shaft u-joints.Another indication of bad u-joints is a vibration at certain speeds (like 65mph).There's play somewhere in the drive train.Check rear suspension too.Jack up back of car so wheels are off the ground,Have someone run the car through a couple gears and look under back of car for exactly where the noise may be.Also while back of car is off the ground w/car off put car in gear so the back wheels dont spin.Grab one of the wheels and rotate it back and forth from stop to stop and listen for any clunk noise.The wheel should turn less than an eighth from stop to stop.