Transmission slips in reverse only

1990 F350 uhaul cube truck. 7.3 diesel c6 trans. I had the trans rebuilt approx 2000 miles ago and it will barely back up an incline. terrible slippage and sometimes aggressive shudders. no problems in any drive gears. The only other odd symptoms are on the first start of the day it will take a few seconds to build line pressure before it will move in any gear. I check fluid level regularly and it is good.

Any advice would be greatly appreciated

Pull the dipstick and smell it if it smells burnt take it back to the rebuild shop.
Pull the pan and check for debris and if noticed take it back. I would take it back and ask the rebuilder what is the problem. Its only been a short time and he should fix it free. Do this before you pull the pan.
Post what what he tells you is going on with it.

take it back to the shop and tell them it ain't working .don't do anything as taking anything apart as the shop will holler that you worked on it and void the warranty. the trans has a warranty on it and the shop has to fix it as the warranty states.if they won't call the better business bureau in your area and they will be glad to fix it.

I guess i should have specified this in the first place but I am more interested in ideas as to what the possible causes are. The transmission has already been back to the shop once because the converter wasn't seated all the way when installed which obviously ate the pump which lead to a total tear down, cleaning and redo, so I wont be taking it back there and I don't have the time and money to go elsewhere because this is my primary vehicle for my construction company. Also I know this transmission has been around for 30 or so years and I was hoping someone throughout the years may have seen a similar problem. Anyways, sorry for the confusion just looking for possible causes. Ill just do the rebuild my self next time if I don't sell the truck first.

Post exactly what transmission you have this might help.

Just a standard C6. Pretty much the same from 1966-1996. The only thing that might be different from a standard f series installation is the big inline filter probably installed by uhaul. All other parts installed during the rebuild came straight from napa.

I had some problems with an AOD after a rebuild. I pulled the valve body and cleaned all of the valves. I found one put in backward. I purchased a paper and rubber gasket kit at transstar. They were a lot cheaper than a parts store.
I was able to get the use of a parts cleaner with new cleaning solution. This is critical when doing transmission work. A good shop will have two parts cleaners one for major cleaning and another for critical items. I had a rebuilder who did good work he quit and started doing them at home. As he didn't have axcess to parts cleaners all jobs came back. This cost me a few dollars.
I think you have some trash in the valve body. I found that using long bolts as guides you can get the gaskets on properly push the valve body upwards and start a couple of the correct length bolts. Then remove the longer bolts.

usually on a ford c-6 trans reverse is the 1st gear to go . it will slip in reverse and move in forward gears but the shuddering could be the bands slipping or valve body losing pressure. the c-6 was a good trans.you could get one rebuilt for $350.00. the trans now cost a lot more to rebuild. i got a rebuilt aod /torque convertor with heavy duty bands and clutches also a heavy duty shift kit in it.it is out of an e-150 van which weighed 5300 pounds and it would pull it like was a sports car. the aod has less than 1000 miles on the rebuild.and the dude that rebuilt it has been doing transmissions for 35 years.i'd let it go for $500.00 and guarantee it even though i have $1200.00 in the trans.the aod shifts by a linkage which would go with it.the aod fits years '82 up to '91. but bottom line whoever rebuilt your c-6 did not know or care about rebuilding one.yeah if the trans was put back together dirty or gaskets in valves then this would cause reverse to loose its pressure and not move.worth a try to look at the valve body but beware of the ball bearings as they got to be in the proper spot.

I would suggest a rebuilders book if you try this. They used to be $15.00 probably more at this time.