Using Newtonsoft.Json in a TFS release definition from powershell - json

Question

Folks,
We are using TFS 2017 on-premise and are working toward provisioning/deploying to an Azure website and database. Our Azure database provisioning/updating task works well, but until we can automate the website portion part, we have an interim step where we copy the web package from the build into a staging share.
This step runs on a Deployment Group under the TFS vstsagent as follows:
Create the temp folder
Expand the zip file into the temp folder
Retrieve the Out folder under the expanded zip file
Move out folder to root of temp folder
Merge the appsettings.json file with the master.appsettings.json file for the environment.
Re-zip the web package
Copy the web package to staging destination share under PRJ\build#
Step 5) is performed using the merge feature of Newtonsoft.Json.
# Load the Newtonsoft.Json dll
$asmNewtonsoftJson = [Reflection.Assembly]::LoadFile([io.path]::combine($FolderToZip, "Newtonsoft.Json.dll"))
# Read the appsettings.json file and store as a JObject
$appSettingsPath = [io.path]::combine($folderToZip, "appsettings.json")
$appSettingsJson = (Get-Content $appSettingsPath | Out-String)
$appSettings = [Newtonsoft.Json.Linq.JObject]::Parse($appSettingsJson)
# Read the master appsettings.json file from the env and store as a JObject
$appSettingsMasterPath = [io.path]::combine($AppSettingsMasterFolderPath, $Environment + ".appsettings.json")
$appSettingsMasterJson = (Get-Content $appSettingsMasterPath | Out-String)
$appSettingsMaster = [Newtonsoft.Json.Linq.JObject]::Parse($appSettingsMasterJson)
# Merge the master appsettings.json file into the appsettings.json file
$jms = New-Object Newtonsoft.Json.Linq.JsonMergeSettings
$jms.MergeArrayHandling = [Newtonsoft.Json.Linq.MergeArrayHandling]::Merge
$appSettings.Merge($appSettingsMaster, $jms)
# Write out the updated appsettings.json file
[io.File]::WriteAllText($appSettingsPath, $appSettings.ToString())
Note: $folderToZip contains the expanded contents of the web package. This also contains the Newtonsoft.Json dll (and presumably its dependencies).
The above code works fine in the Windows Powershell ISE app, but fails when run in the TFS release definition task.
It fails at step 5) with the following error:
2018-10-11T19:29:39.1790546Z ##[error]Unable to find type [Newtonsoft.Json.Linq.JObject].
At C:\vstsagent\A1\_work\r7\a\<path>\drop\Build\mymergescript.ps1:84 char:20
$appSettings = [Newtonsoft.Json.Linq.JObject]::Parse($appSettings)
CategoryInfo : InvalidOperation: (Newtonsoft.Json.Linq.JObject:TypeName) [], RuntimeException
FullyQualifiedErrorId : TypeNotFound
It appears that the Newtonsoft.Json.dll isn't getting loaded properly when deployed by the TFS vstsagent.
Any help here is appreciated.