Driveline resistance with 2001 Cobra IRS

I have a 1993 GT, and I have been going through the steps of an IRS swap over the past several months, and finally drove the car about 1 mile. The problem now is that there seems to be quite a lot of driveline resistance. I have thoroughly cleaned out the diff and have fresh diff oil in there. I have the full set of FTBR bushings for the IRS. I even have all of the FTBR differential mounting hardware. The bearings in the diff for the axles are new. Here are some observances:

1. With the back of the car in the air (suspension is full droop) and the car in 1st gear, when I try to just let the clutch out slowly with no gas, it will stall. If I give it enough gas, it can spin the rear wheels and once the clutch is fully engaged, the rear wheels spin without any extra gas.
2. When on the road, starting from a stop in first gear takes more gas than I think is necessary (seems to match item 1). Once I am moving though, and I just coast, I don't grind to a halt as I would expect if there was a lot of driveline resistance. Although, maybe this is just because the kinetic energy of this heavy car moving makes the driveline resistance seem negligible.

Any ideas of what I need to check? I don't think I ever actually replaced the wheel bearings for the rears (I only replaced the bearings in the diff). I have no idea what angles my driveshaft/transmission/differential are at. Thanks ahead of time for the help.

Mustang5L5 said:

Rear brakes grabbing/dragging? If you jack up the rear and put the trans in neutral, can you easily spin the rear wheels and driveshaft?

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I'll try tonight to see if the rear brakes are dragging. Thanks for the tip.

Thanks. I am using the stock control arms that came with my IRS. I'll report back this evening or tomorrow with results.

Problem seems not as bad now. I lifted the rear and was able to rotate the wheels by hand. I did hear the brake disk rubbing against the pads a little bit though. The brakes are all new, so maybe they just need to be worn in? I did the test again where the rear is raised, and I put the car in 1st and just slowly let the clutch out (no gas), and it still struggled. Is that even a legitimate test? Maybe just that little bit of brake resistance gives it a hard time? Anyway, I will say it is probably resolved, and will keep an eye on it.

Thanks.

That's a good point. But if I can turn the wheels by hand, that means from drive shaft to the wheels are probably OK right? That would leave the transmission/clutch? What do you think I should try next?