truck wont go in drive or reverse
 

Hey everyone, newbie to the forum! Got an 05 chevy 2500 hd with the Allison trans. 150k miles. Had a snow storm 3 days ago, so the 4x4 got worked hard for the first time in awhile. Then was parked for a day and a half. Jump in the truck this morning, put er in reverse and nothing, go for drive and nothing, same goes for all the gears. Switched between 4H and L, still nothing. The CEL is throwing these codes..P0700, P0843, P0841. Did some digging and came up with these possible causes.. low trans fluid, spin on filter needs changing, internal valves are stuck, or the nsbu switch needs replacing. The dash lights for the gear selector burned out years ago, so I cant tell if the bar is flashing to indicate bad nsbu. Trans fluid level was checked a couple months ago and was good, but is in need of changing. Seems to me that the snow storm may have done the nsbu in, so picking one up tomorrow. Unfortunately its Detroit diesel and not the Allison replacement. Hopefully that's not going to be a problem. Anyone have any experience with this problem? Any advice would be greatly appreciated! Thanks in advance!